1. Explain the difference between a hot backup and a cold backup and the benefits associated with each.

A hot backup is basically taking a backup of the database while it is still up and running and it must be in archive log mode. A cold backup is taking a backup of the database while it is shut down and does not require being in archive log mode. The benefit of taking a hot backup is that the database is still available for use while the backup is occurring and you can recover the database to any point in time. The benefit of taking a cold backup is that it is typically easier to administer the backup and recovery process.

2. You have just had to restore from backup and do not have any control files. How would you go about bringing up this database?

I would create a text based backup control file, stipulating اشتراط where on disk all the data files were and then issue the recover command with the using backup control file clause.

3. How do you switch from an init.ora file to a spfile? Issue the create spfile from pfile command.

4. Explain the difference between a data block, an extent and a segment.

A data block is the smallest unit of logical storage for a database object. As objects grow they take chunks of additional storage that are composed of contiguous data blocks. These groupings of contiguous data blocks are called extents. All the extents that an object takes when grouped together are considered the segment of the database object.

5. Give two examples of how you might determine the structure of the table DEPT.

Use the describe command or use the dbms_metadata.get_ddl package.

        SELECT DBMS_METADATA.GET_DDL('TABLE','MERCHANT_MACHINES') FROM dual;

6. Where would you look for errors from the database engine? In the alert log.

7. Compare and contrast TRUNCATE and DELETE for a table.

Both the truncate and delete command delete rows in a table. The differ is that truncate command is a DDL (Data Definition Language ) operation and just moves the high water mark and produces few rollback data. The delete command, on the other hand, is a DML, which will produce rollback data and thus take longer to complete.

8. Give the reasoning behind using an index. Faster access to data blocks in a table.

9. Give the two types of tables involved in producing a star schema and the type of data they hold.

Fact tables and dimension tables. A fact table contains measurements while dimension tables will contain data that will help describe the fact tables.

10. What type of index should you use on a fact table? A Bitmap index.

11. Give some examples of the types of database contraints you may find in Oracle and indicate their purpose.

· A Primary or Unique Key can be used to enforce uniqueness on one or more columns.

· A Referential Integrity Contraint can be used to enforce a Foreign Key relationship between two tables.

· A Not Null constraint - to ensure a value is entered in a column

· A Value Constraint - to check a column value against a specific set of values.

12. A table is classified as a parent table and you want to drop and re-create it. How would you do this without affecting the children tables?

Disable the foreign key constraint to the parent, drop the table, re-create the table, enable the foreign key constraint.

13. Explain the difference between ARCHIVELOG mode and NOARCHIVELOG mode and the benefits and disadvantages to each.

ARCHIVELOG mode is a mode that you can put the database in for creating a backup of all transactions that have occurred in the database so that you can recover to any point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any point in time. NOARCHIVELOG mode does have the advantage of not having to write transactions to an archive log and thus increases the performance of the database slightly.

14. What command would you use to create a backup control file? Alter database backup control file to trace.

15. Give the stages of instance startup to a usable state where normal users may access it.

STARTUP NOMOUNT - Instance startup

STARTUP MOUNT - The database is mounted

STARTUP OPEN - The database is opened

16. What column differentiates the V$ views to the GV$ views and how?

V$ is for single instance. gv$ is for RAC; all instances.

Oracle RAC allows multiple computers to run Oracle RDBMS software simultaneously while accessing a single database, thus providing clustering. In a non-RAC Oracle database, a single instance accesses a single database. The database consists of a collection of data files, control files, and redo logs located on disk. The instance comprises the collection of Oracle-related memory and operating system processes that run on a computer system.

17. How would you go about generating an EXPLAIN plan?

Create a plan table with utlxplan.sql. Use the explain plan set statement_id = 'tst1' into plan_table for a SQL statement Look at the explain plan with utlxplp.sql or utlxpls.sql

18. How would you go about increasing the buffer cache hit ratio?

Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command.

19. Explain an ORA-01555.

You get this error when you get a snapshot too old within rollback. It can usually be solved by increasing the undo retention or increasing the size of rollbacks. You should also look at the logic involved in the application getting the error message.

20. Explain the difference between $ORACLE_HOME and $ORACLE_BASE.

ORACLE_BASE - The home directory for the Oracle software (e.g. /u01/app/oracle/product/10.2.1) with subdirectories like:

bin – rdbms – sqlplus - etc.

ORACLE_HOME - Below ORACLE_BASE, the files for each database: Oracle Interview Questions

1. How would you determine the time zone under which a database was operating?

SELECT dbtimezone FROM DUAL; SELECT TZ_OFFSET('America/Menominee') FROM DUAL;

select sessiontimezone from dual;

2. Explain the use of setting GLOBAL_NAMES equal to TRUE.

It ensure the use of consistent naming conventions for databases and links in a networked environment.

3. What command would you use to encrypt a PL/SQL application? WRAP

4. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.

They are all named PL/SQL blocks.

Function must return a value. Can be called inside a query.

Procedure may or may not return value.

Package is the collection of functions, procedures, variables which can be logically grouped together.

9. When a user process fails, what background process cleans up after it? PMON

10. What background process refreshes materialized views? Job Queue Process (CJQ)

11. How would you determine what sessions are connected and what resources they are waiting for?

v$session,v$session_wait

13. How would you force a log switch? alter system switch logfile;

15. What does coalescing a tablespace do?

Coalesce simply takes contigous free extents and makes them into a single bigger free extent.

16. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?

TEMP tablespace gets cleared once the transaction is done where as PERMANENT tablespace retails the data.

17. Name a tablespace automatically created when you create a database. SYSTEM

18. When creating a user, what permissions must you grant to allow them to connect to the database?

Grant create session to username;

19. How do you add a data file to a tablespace?

alter tablespace USERS add datafile '/ora01/oradata/users02.dbf' size 50M;

20. How do you resize a data file?

alter database datafile '/ora01/oradata/users02.dbf' resize 100M;

21. What view would you use to look at the size of a data file?

dba_data_files

22. What view would you use to determine free space in a tablespace?

dba_free_space

select * from dba_free_space where TABLESPACE_NAME = 'BASIC_DATA';

23. How would you determine who has added a row to a table?

By implementing an INSERT trigger for logging details during each INSERT operation on the table

select * from all_objects ao where ao.OBJECT_TYPE = 'TABLE' and ao.OWNER = 'ALPHA'

24. How can you rebuild an index? ALTER INDEX index_name REBUILD;

25. Explain what partitioning is and what its benefit is.

A table partition is also a table segment, and by using partitioning technique we can enhance performance of table access.

26. You have just compiled a PL/SQL package but got errors, how would you view the errors? show errors

27. How can you gather statistics on a table? exec dbms_stats.gather_table_stats

EXEC DBMS_STATS.gather_table_stats('ALPHA', 'test2');

Also, remember to analyze all associated indexes on that table using dbms_stats.gather_index_stats

28. How can you enable a trace for a session? alter session set sql_trace='TRUE';

29. What is the difference between the SQL*Loader and IMPORT utilities?

SQL*LOADER loads external data which is in OS files to oracle database tables while IMPORT utility imports data only which is exported by EXPORT utility of oracle database.

30. Name two files used for network connection to a database. TNSNAMES.ORA and SQLNET.ORA

2. What is a mutating table error and how can you get around it? This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.

3. Describe the use of %ROWTYPE and %TYPE in PL/SQL Expected answer: %ROWTYPE allows you to associate a variable with an entire table row. The %TYPE associates a variable with a single column type.

4. What packages (if any) has Oracle provided for use by developers? Expected answer: Oracle provides the DBMS_ series of packages. There are many which developers should be aware of such as DBMS_SQL, DBMS_PIPE The DBMS_PIPE package lets two or more sessions in the same instance communicate., DBMS_TRANSACTION, DBMS_LOCK The DBMS_LOCK package provides an interface to Oracle Lock Management services., DBMS_ALERT DBMS_ALERT supports asynchronous notification of database events (alerts)., DBMS_OUTPUT The DBMS_OUTPUT package enables you to send messages from stored procedures, DBMS_JOB DBMS_JOB subprograms schedule and manage jobs in the job queue., DBMS_UTILITY The DBMS_UTILITY package provides various utility subprograms, DBMS_DDL This package provides access to some SQL data definition language (DDL) statements from stored procedures. It also provides special administration operations that are not available as Data Definition Language statements (DDLs). , UTL_FILE With the UTL_FILE package, PL/SQL programs can read and write operating system text files. UTL_FILE provides a restricted version of operating system stream file I/O..
5. Describe the use of PL/SQL tables Expected answer: PL/SQL tables are scalar arrays that can be referenced by a binary integer. They can be used to hold values for use in later queries or calculations. In Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.

6. When is a declare statement needed ? The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.

7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the %NOTFOUND cursor variable in the exit when statement? Why? Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.

8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers? Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.

DECLARE

   name employees.last_name%TYPE;

   v_code NUMBER;

   v_errm VARCHAR2(64);

BEGIN

   SELECT last_name INTO name FROM employees WHERE employee_id = 1000;

   EXCEPTION

      WHEN OTHERS THEN

         v_code := SQLCODE;

         v_errm := SUBSTR(SQLERRM, 1 , 64);

         DBMS_OUTPUT.PUT_LINE('The error code is ' || v_code || '- ' || v_errm);

END;

9. How can you find within a PL/SQL block, if a cursor is open? Use the %ISOPEN cursor status variable.

10. How can you generate debugging output from PL/SQL? Use the DBMS_OUTPUT package. command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can also be used.

11. What are the types of triggers? BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words: BEFORE ALL ROW INSERT AFTER ALL ROW INSERT BEFORE INSERT AFTER INSERT etc.

1. A tablespace has a table with 30 extents in it. Is this bad? Why or why not.

Multiple extents in and of themselves aren?t bad. However if you also have chained rows this can hurt performance.

2. How do you set up tablespaces during an Oracle installation?

You should always attempt to use the Oracle Flexible Architecture standard or another partitioning scheme to ensure proper separation of SYSTEM, ROLLBACK, REDO LOG, DATA, TEMPORARY and INDEX segments.

3. You see multiple fragments in the SYSTEM tablespace, what should you check first?

Ensure that users don?t have the SYSTEM tablespace as their TEMPORARY or DEFAULT tablespace assignment by checking the DBA_USERS view.

4. What are some indications that you need to increase the SHARED_POOL_SIZE parameter? Poor data dictionary or library cache hit ratios, getting error ORA-04031. Another indication is steadily decreasing performance with all other tuning parameters the same.

5. What is the general guideline for sizing db_block_size and db_multi_block_read for an application that does many full table scans?

Oracle almost always reads in 64k chunks. The two should have a product equal to 64 or a multiple of 64.

6. What is the fastest query method for a table Fetch by rowid

7. Explain the use of TKPROF? What initialization parameter should be turned on to get full TKPROF output?

The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.

8. When looking at v$sysstat you see that sorts (disk) is high. Is this bad or good? If bad -How do you correct it?

If you get excessive disk sorts this is bad. This indicates you need to tune the sort area parameters in the initialization files. The major sort are parameter is the SORT_AREA_SIZe parameter.

9. When should you increase copy latches? What parameters control copy latches

When you get excessive contention for the copy latches as shown by the "redo copy" latch hit ratio. You can increase copy latches via the initialization parameter LOG_SIMULTANEOUS_COPIES to twice the number of CPUs on your system.

10. Where can you get a list of all initialization parameters for your instance? How about an indication if they are default settings or have been changed

You can look in the init.ora file for an indication of manually set parameters. For all parameters, their value and whether or not the current value is the default value, look in the v$parameter view.

11. Describe hit ratio as it pertains to the database buffers. What is the difference between instantaneous and cumulative hit ratio and which should be used for tuning

The hit ratio is a measure of how many times the database was able to read a value from the buffers verses how many times it had to re-read a data value from the disks. A value greater than 80-90% is good, less could indicate problems. If you simply take the ratio of existing parameters this will be a cumulative value since the database started. If you do a comparison between pairs of readings based on some arbitrary time span, this is the instantaneous ratio for that time span. Generally speaking an instantaneous reading gives more valuable data since it will tell you what your instance is doing for the time it was generated over.

12. Discuss row chaining, how does it happen? How can you reduce it? How do you correct it

Row chaining occurs when a VARCHAR2 value is updated and the length of the new value is longer than the old value and won?t fit in the remaining block space. This results in the row chaining to another block. It can be reduced by setting the storage parameters on the table to appropriate values. It can be corrected by export and import of the effected table.

1. Give one method for transferring a table from one schema to another:

There are several possible methods, export-import, CREATE TABLE... AS SELECT, or COPY.

2. What is the purpose of the IMPORT option IGNORE? What is it?s default setting The IMPORT IGNORE option tells import to ignore "already exists" errors. If it is not specified the tables that already exist will be skipped. If it is specified, the error is ignored and the tables data will be inserted. The default value is N.

3. You have a rollback segment in a version 7.2 database that has expanded beyond optimal, how can it be restored to optimal Use the ALTER TABLESPACE ..... SHRINK command.

4. If the DEFAULT and TEMPORARY tablespace clauses are left out of a CREATE USER command what happens? Is this bad or good? Why

The user is assigned the SYSTEM tablespace as a default and temporary tablespace. This is bad because it causes user objects and temporary segments to be placed into the SYSTEM tablespace resulting in fragmentation and improper table placement (only data dictionary objects and the system rollback segment should be in SYSTEM).

5. What are some of the Oracle provided packages that DBAs should be aware of

Oracle provides a number of packages in the form of the DBMS_ packages owned by the SYS user. The packages used by DBAs may include: DBMS_SHARED_POOL, DBMS_UTILITY, DBMS_SQL, DBMS_DDL, DBMS_SESSION, DBMS_OUTPUT and DBMS_SNAPSHOT. They may also try to answer with the UTL*.SQL or CAT*.SQL series of SQL procedures. These can be viewed as extra credit but aren?t part of the answer.

6. What happens if the constraint name is left out of a constraint clause

The Oracle system will use the default name of SYS_Cxxxx where xxxx is a system generated number. This is bad since it makes tracking which table the constraint belongs to or what the constraint does harder.

7. What happens if a tablespace clause is left off of a primary key constraint clause

This results in the index that is automatically generated being placed in then users default tablespace. Since this will usually be the same tablespace as the table is being created in, this can cause serious performance problems.

8. What is the proper method for disabling and re-enabling a primary key constraint

You use the ALTER TABLE command for both. However, for the enable clause you must specify the USING INDEX and TABLESPACE clause for primary keys.

9. What happens if a primary key constraint is disabled and then enabled without fully specifying the index clause

The index is created in the user?s default tablespace and all sizing information is lost. Oracle doesn?t store this information as a part of the constraint definition, but only as part of the index definition, when the constraint was disabled the index was dropped and the information is gone.

10. (On UNIX) When should more than one DB writer process be used? How many should be used

If the UNIX system being used is capable of asynchronous IO then only one is required, if the system is not capable of asynchronous IO then up to twice the number of disks used by Oracle number of DB writers should be specified by use of the db_writers initialization parameter.

11. You are using hot backup without being in archivelog mode, can you recover in the event of a failure? Why or why not

You can?t use hot backup without being in archivelog mode. So no, you couldn?t recover.

12. What causes the "snapshot too old" error? How can this be prevented or mitigated

This is caused by large or long running transactions that have either wrapped onto their own rollback space or have had another transaction write on part of their rollback space. This can be prevented or mitigated by breaking the transaction into a set of smaller transactions or increasing the size of the rollback segments and their extents.

13. How can you tell if a database object is invalid By checking the status column of the DBA_, ALL_ or USER_OBJECTS views, depending upon whether you own or only have permission on the view or are using a DBA account.

13. A user is getting an ORA-00942 error yet you know you have granted them permission on the table, what else should you check

You need to check that the user has specified the full name of the object (select empid from scott.emp; instead of select empid from emp;) or has a synonym that balls to the object (create synonym emp for scott.emp;)

14. A developer is trying to create a view and the database won?t let him. He has the "DEVELOPER" role which has the "CREATE VIEW" system privilege and SELECT grants on the tables he is using, what is the problem

You need to verify the developer has direct grants on all tables used in the view. You can?t create a stored object with grants given through views.

15. If you have an example table, what is the best way to get sizing data for the production table implementation

The best way is to analyze the table and then use the data provided in the DBA_TABLES view to get the average row length and other pertinent data for the calculation. The quick and dirty way is to look at the number of blocks the table is actually using and ratio the number of rows in the table to its number of blocks against the number of expected rows.

16. How can you find out how many users are currently logged into the database? How can you find their operating system id

There are several ways. One is to look at the v$session or v$process views. Another way is to check the current_logins parameter in the v$sysstat view. Another if you are on UNIX is to do a "ps -ef|grep oracle|wc -l? command, but this only works against a single instance installation.

17. A user selects from a sequence and gets back two values, his select is: SELECT pk_seq.nextval FROM dual;What is the problem Somehow two values have been inserted into the dual table. This table is a single row, single column table that should only have one value in it.

18. How can you determine if an index needs to be dropped and rebuilt

Run the ANALYZE INDEX command on the index to validate its structure and then calculate the ratio of LF_BLK_LEN/LF_BLK_LEN+BR_BLK_LEN and if it isn?t near 1.0 (i.e. greater than 0.7 or so) then the index should be rebuilt. Or if the ratio BR_BLK_LEN/ LF_BLK_LEN+BR_BLK_LEN is nearing 0.3.

1. How can variables be passed to a SQL routine

By use of the & symbol. For passing in variables the numbers 1-8 can be used (&1, &2,...,&8) to pass the values after the command into the SQLPLUS session. To be prompted for a specific variable, place the ampersanded variable in the code itself: "select * from dba_tables where owner=&owner_name;" . Use of double ampersands tells SQLPLUS to resubstitute the value for each subsequent use of the variable, a single ampersand will cause a reprompt for the value unless an ACCEPT statement is used to get the value from the user.

2. You want to include a carriage return/linefeed in your output from a SQL script, how can you do this

The best method is to use the CHR() function (CHR(10) is a return/linefeed) and the concatenation function "||". Another method, although it is hard to document and isn?t always portable is to use the return/linefeed as a part of a quoted string.

3. How can you call a PL/SQL procedure from SQL By use of the EXECUTE (short form EXEC) command.

4. How do you execute a host operating system command from within SQL

By use of the exclamation ball "!" (in UNIX and some other OS) or the HOST (HO) command.

5. You want to use SQL to build SQL, what is this called and give an example

This is called dynamic SQL. An example would be: set lines 90 pages 0 termout off feedback off verify off spool drop_all.sql select ?drop user ?||username||? cascade;? from dba_users where username not in ("SYS?,?SYSTEM?); spool off Essentially you are looking to see that they know to include a command (in this case DROP USER...CASCADE;) and that you need to concatenate using the ?||? the values selected from the database.

6. What SQLPlus command is used to format output from a select This is best done with the COLUMN command.

7. You want to group the following set of select returns, what can you group on

Max(sum_of_cost), min(sum_of_cost), count(item_no), item_no The only column that can be grouped on is the "item_no" column, the rest have aggregate functions associated with them.

8. What special Oracle feature allows you to specify how the cost based system treats a SQL statement

The COST based system allows the use of HINTs to control the optimizer path selection. If they can give some example hints such as FIRST ROWS, ALL ROWS, USING INDEX, STAR, even better.

9. You want to determine the location of identical rows in a table before attempting to place a unique index on the table, how can this be done

Oracle tables always have one guaranteed unique column, the rowid column. If you use a min/max function against your rowid and then select against the proposed primary key you can squeeze out the rowids of the duplicate rows pretty quick. For example: select rowid from emp e where e.rowid > (select min(x.rowid) from emp x where x.emp_no = e.emp_no); In the situation where multiple columns make up the proposed key, they must all be used in the where clause.

10. What is a Cartesian product A Cartesian product is the result of an unrestricted join of two or more tables. The result set of a three table Cartesian product will have x * y * z number of rows where x, y, z correspond to the number of rows in each table involved in the join.

Joins

A join is a query that combines rows from two or more tables, views, or materialized views. Oracle Database performs a join whenever multiple tables appear in the FROM clause of the query. The select list of the query can select any columns from any of these tables. If any two of these tables have a column name in common, then you must qualify all references to these columns throughout the query with table names to avoid ambiguity.

Join Conditions

Most join queries contain at least one join condition, either in the FROM clause or in the WHERE clause. The join condition compares two columns, each from a different table. To execute a join, Oracle Database combines pairs of rows, each containing one row from each table, for which the join condition evaluates to TRUE. The columns in the join conditions need not also appear in the select list.

To execute a join of three or more tables, Oracle first joins two of the tables based on the join conditions comparing their columns and then joins the result to another table based on join conditions containing columns of the joined tables and the new table. Oracle continues this process until all tables are joined into the result. The optimizer determines the order in which Oracle joins tables based on the join conditions, indexes on the tables, and, any available statistics for the tables.

IA WHERE clause that contains a join condition can also contain other conditions that refer to columns of only one table. These conditions can further restrict the rows returned by the join query.

Note:

You cannot specify LOB columns in the WHERE clause if the WHERE clause contains the join condition. The use of LOBs in WHERE clauses is also subject to other restrictions. See Oracle Database SecureFiles and Large Objects Developer's Guide for more information.

Equijoins

An equijoin is a join with a join condition containing an equality operator. An equijoin combines rows that have equivalent values for the specified columns. Depending on the internal algorithm the optimizer chooses to execute the join, the total size of the columns in the equijoin condition in a single table may be limited to the size of a data block minus some overhead. The size of a data block is specified by the initialization parameter DB_BLOCK_SIZE.

Self Joins

A self join is a join of a table to itself. This table appears twice in the FROM clause and is followed by table aliases that qualify column names in the join condition. To perform a self join, Oracle Database combines and returns rows of the table that satisfy the join condition.

Cartesian Products

If two tables in a join query have no join condition, then Oracle Database returns their Cartesian product. Oracle combines each row of one table with each row of the other. A Cartesian product always generates many rows and is rarely useful. For example, the Cartesian product of two tables, each with 100 rows, has 10,000 rows. Always include a join condition unless you specifically need a Cartesian product. If a query joins three or more tables and you do not specify a join condition for a specific pair, then the optimizer may choose a join order that avoids producing an intermediate Cartesian product.

Inner Joins

An inner join (sometimes called a simple join) is a join of two or more tables that returns only those rows that satisfy the join condition.

Outer Joins

An outer join extends the result of a simple join. An outer join returns all rows that satisfy the join condition and also returns some or all of those rows from one table for which no rows from the other satisfy the join condition.

· To write a query that performs an outer join of tables A and B and returns all rows from A (a left outer join), use the LEFT [OUTER] JOIN syntax in the FROM clause, or apply the outer join operator (+) to all columns of B in the join condition in the WHERE clause. For all rows in A that have no matching rows in B, Oracle Database returns null for any select list expressions containing columns of B.

· To write a query that performs an outer join of tables A and B and returns all rows from B (a right outer join), use the RIGHT [OUTER] JOIN syntax in the FROM clause, or apply the outer join operator (+) to all columns of A in the join condition in the WHERE clause. For all rows in B that have no matching rows in A, Oracle returns null for any select list expressions containing columns of A.

· To write a query that performs an outer join and returns all rows from A and B, extended with nulls if they do not satisfy the join condition (a full outer join), use the FULL [OUTER] JOIN syntax in the FROM clause.

You cannot compare a column with a subquery in the WHERE clause of any outer join, regardless which form you specify. You can use outer joins to fill gaps in sparse data. Such a join is called a partitioned outer join and is formed using the query_partition_clause of the join_clause syntax. Sparse data is data that does not have rows for all possible values of a dimension such as time or department. For example, tables of sales data typically do not have rows for products that had no sales on a given date. Filling data gaps is useful in situations where data sparsity complicates analytic computation or where some data might be missed if the sparse data is queried directly.

Antijoins

An antijoin returns rows from the left side of the predicate for which there are no corresponding rows on the right side of the predicate. It returns rows that fail to match (NOT IN) the subquery on the right side.

Semijoins

A semijoin returns rows that match an EXISTS subquery without duplicating rows from the left side of the predicate when multiple rows on the right side satisfy the criteria of the subquery.

Semijoin and antijoin transformation cannot be done if the subquery is on an OR branch of the WHERE clause.

INNER Versus OUTER Joins

• In SQL:1999, the join of two tables returning only matched rows is called an inner join.

• A join between two tables that returns the results of the inner join as well as the unmatched rows from the left (or right) tables is called a left (or right) outer join.

• A join between two tables that returns the results of an inner join as well as the results of a left and right join is a full outer join.

11. You are joining a local and a remote table, the network manager complains about the traffic involved, how can you reduce the network traffic Push the processing of the remote data to the remote instance by using a view to pre-select the information for the join. This will result in only the data required for the join being sent across.

11. What is the default ordering of an ORDER BY clause in a SELECT statement

Ascending

12. What is tkprof and how is it used

13. What is explain plan and how is it used

The EXPLAIN PLAN command is a tool to tune SQL statements. To use it you must have an explain_table generated in the user you are running the explain plan for. This is created using the utlxplan.sql script. Once the explain plan table exists you run the explain plan command giving as its argument the SQL statement to be explained. The explain_plan table is then queried to see the execution plan of the statement. Explain plans can also be run using tkprof.

14. How do you set the number of lines on a page of output? The width

The SET command in SQLPLUS is used to control the number of lines generated per page and the width of those lines, for example SET PAGESIZE 60 LINESIZE 80 will generate reports that are 60 lines long with a line width of 80 characters. The PAGESIZE and LINESIZE options can be shortened to PAGES and LINES.

15. How do you prevent output from coming to the screen

The SET option TERMOUT controls output to the screen. Setting TERMOUT OFF turns off screen output. This option can be shortened to TERM.

16. How do you prevent Oracle from giving you informational messages during and after a SQL statement execution

The SET options FEEDBACK and VERIFY can be set to OFF.

17. How do you generate file output from SQL

By use of the SPOOL comm

1. What is a CO-RELATED SUBQUERY

A CO-RELATED SUBQUERY is one that has a correlation name as table or view designator in the FROM clause of the outer query and the same correlation name as a qualifier of a search condition in the WHERE clause of the subquery. eg

    SELECT  field1 from table1 X

    WHERE  field2>(select avg(field2) from table1 Y

                                      where

                                      field1=X.field1);

(The subquery in a correlated subquery is revaluated for every row of the table or view named in the outer query.)

2. What are various joins used while writing SUBQUERIES

Self join-Its a join foreign key of a table references the same table.

Outer Join--Its a join condition used where One can query all the rows of one of the tables in the join condition even though they don't satisfy the join condition.

Equi-join--Its a join condition that retrieves rows from one or more tables in which one or more columns in one table are equal to one or more columns in the second table.

3. What are various constraints used in SQL

NULL NOT NULL CHECK DEFAULT

4. What are different Oracle database objects

TABLES VIEWS INDEXES SYNONYMS SEQUENCES TABLESPACES etc

5. What is difference between Rename and Alias

Rename is a permanent name given to a table or column whereas Alias is a temporary name given to a table or column which do not exist once the SQL statement is executed.

6. What is a view

A view is stored procedure based on one or more tables, its a virtual table.

7. What are various privileges that a user can grant to another user

SELECT CONNECT RESOURCE

8. What is difference between UNIQUE and PRIMARY KEY constraints

A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL.

9. Can a primary key contain more than one columns

Yes

10. How you will avoid duplicating records in a query

By using DISTINCT

11. What is difference between SQL and SQL*PLUS

SQL*PLUS is a command line tool where as SQL and PL/SQL language interface and reporting tool. Its a command line tool that allows user to type SQL commands to be executed directly against an Oracle database. SQL is a language used to query the relational database(DML,DCL,DDL). SQL*PLUS commands are used to format query result, Set options, Edit SQL commands and PL/SQL.

12. Which datatype is used for storing graphics and images

LONG RAW data type is used for storing BLOB's (binary large objects).

13. How will you delete duplicating rows from a base table

DELETE FROM table_name A WHERE rowid>(SELECT min(rowid) from table_name B where B.table_no=A.table_no);

CREATE TABLE new_table AS SELECT DISTINCT * FROM old_table;

DROP old_table RENAME new_table TO old_table DELETE FROM table_name A WHERE rowid NOT IN (SELECT MAX(ROWID) FROM table_name GROUP BY column_name)

14. What is difference between SUBSTR and INSTR

SUBSTR returns a specified portion of a string eg SUBSTR('BCDEF',4) output BCDE INSTR provides character position in which a pattern is found in a string.

eg INSTR('ABC-DC-F','-',2) output 7 (2nd occurence of '-')

15. There is a string '120000 12 0 .125' ,how you will find the position of the decimal place

INSTR('120000 12 0 .125','.',1) output 13

16. There is a '%' sign in one field of a column. What will be the query to find it.

'\' Should be used before '%'.

17. When you use WHERE clause and when you use HAVING clause

HAVING clause is used when you want to specify a condition for a group function and it is written after GROUP BY clause The WHERE clause is used when you want to specify a condition for columns, single row functions except group functions and it is written before GROUP BY clause if it is used.

18. Which is more faster - IN or EXISTS

EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.

Appropriate answer will be....

Result of the subquery is small Then "IN" is typicaly more appropriate. and Result of the subquery is big/large/long Then "EXIST" is more appropriate.

19. What is a OUTER JOIN

Outer Join--Its a join condition used where you can query all the rows of one of the tables in the join condition even though they dont satisfy the join condition.

20. How you will avoid your query from using indexes

SELECT * FROM emp Where emp_no+' '=12345;

i.e you have to concatenate the column name with space within codes in the where condition.

SELECT /*+ FULL(a) */ ename, emp_no from emp where emp_no=1234;

i.e using HINTS

Oracle Interview Questions

1. What is a pseudo column. Give some examples

It is a column that is not an actual column in the table.

eg USER, UID, SYSDATE, ROWNUM, ROWID, NULL, AND LEVEL.

Suppose customer table is there having different columns like customer no, payments.What will be the query to select top three max payments.

For top N queries, see http://www.orafaq.com/forum/mv/msg/160920/472554/102589/#msg_472554 post

2. What is the purpose of a cluster.

Oracle does not allow a user to specifically locate tables, since that is a part of the function of the RDBMS. However, for the purpose of increasing performance, oracle allows a developer to create a CLUSTER. A CLUSTER provides a means for storing data from different tables together for faster retrieval than if the table placement were left to the RDBMS.

3. What is a cursor.

Oracle uses work area to execute SQL statements and store processing information PL/SQL construct called a cursor lets you name a work area and access its stored information A cursor is a mechanism used to fetch more than one row in a Pl/SQl block.

4. Difference between an implicit & an explicit cursor.

PL/SQL declares a cursor implicitly for all SQL data manipulation statements, including quries that return only one row. However,queries that return more than one row you must declare an explicit cursor or use a cursor FOR loop.

Explicit cursor is a cursor in which the cursor name is explicitly assigned to a SELECT statement via the CURSOR...IS statement. An implicit cursor is used for all SQL statements Declare, Open, Fetch, Close. An explicit cursors are used to process multirow SELECT statements An implicit cursor is used to process INSERT, UPDATE, DELETE and single row SELECT. .INTO statements.

5. What are cursor attributes

%ROWCOUNT %NOTFOUND %FOUND %ISOPEN

6. What is a cursor for loop.

Cursor For Loop is a loop where oracle implicitly declares a loop variable, the loop index that of the same record type as the cursor's record.

7. Difference between NO DATA FOUND and %NOTFOUND

NO DATA FOUND is an exception raised only for the SELECT....INTO statements when the where clause of the querydoes not match any rows. When the where clause of the explicit cursor does not match any rows the %NOTFOUND attribute is set to TRUE instead.

8. What a SELECT FOR UPDATE cursor represent.

SELECT......FROM......FOR......UPDATE[OF column-reference][NOWAIT] The processing done in a fetch loop modifies the rows that have been retrieved by the cursor. A convenient way of modifying the rows is done by a method with two parts: the FOR UPDATE clause in the cursor declaration, WHERE CURRENT OF CLAUSE in an UPDATE or declaration statement.

9. What 'WHERE CURRENT OF ' clause does in a cursor.

LOOP

                         SELECT  num_credits  INTO  v_numcredits  FROM classes

                         WHERE  dept=123 and course=101;

                         UPDATE  students

                         SET current_credits=current_credits+v_numcredits

                         WHERE  CURRENT OF  X;

                         END  LOOP

                         COMMIT;

                         END;

10. What is use of a cursor variable? How it is defined.

A cursor variable is associated with different statements at run time, which can hold different values at run time. Static cursors can only be associated with one run time query. A cursor variable is reference type(like a pointer in C). Declaring a cursor variable: TYPE type_name IS REF CURSOR RETURN return_type type_name is the name of the reference type,return_type is a record type indicating the types of the select list that will eventually be returned by the cursor variable.

11. What should be the return type for a cursor variable.Can we use a scalar data type as return type.

The return type for a cursor must be a record type.It can be declared explicitly as a user-defined or %ROWTYPE can be used. eg TYPE t_studentsref IS REF CURSOR RETURN students%ROWTYPE

12. How you open and close a cursor variable.Why it is required.

OPEN cursor variable FOR SELECT...Statement CLOSE cursor variable In order to associate a cursor variable with a particular SELECT statement OPEN syntax is used.In order to free the resources used for the query CLOSE statement is used.

13. How you were passing cursor variables in PL/SQL 2.2.

In PL/SQL 2.2 cursor variables cannot be declared in a package.This is because the storage for a cursor variable has to be allocated using Pro*C or OCI with version 2.2,the only means of passing a cursor variable to a PL/SQL block is via bind variable or a procedure parameter.

14. Can cursor variables be stored in PL/SQL tables.If yes how.If not why.

No, a cursor variable points a row which cannot be stored in a two-dimensional PL/SQL table.

15. Difference between procedure and function.

Functions are named PL/SQL blocks that return a value and can be called with arguments procedure a named block that can be called with parameter. A procedure all is a PL/SQL statement by itself, while a Function call is called as part of an expression.

16. What are different modes of parameters used in functions and procedures.

IN OUT INOUT

17. What is difference between a formal and an actual parameter

The variables declared in the procedure and which are passed, as arguments are called actual, the parameters in the procedure declaration. Actual parameters contain the values that are passed to a procedure and receive results. Formal parameters are the placeholders for the values of actual parameters

18. Can the default values be assigned to actual parameters.

Yes

19. Can a function take OUT parameters.If not why.

Yes. A function return a value, but can also have one or more OUT parameters. it is best practice, however to use a procedure rather than a function if you have multiple values to return.

20. What is syntax for dropping a procedure and a function .Are these operations possible.

              Drop Procedure procedure_name

              Drop Function function_name

21. What are ORACLE PRECOMPILERS.

Using ORACLE PRECOMPILERS ,SQL statements and PL/SQL blocks can be contained inside 3GL programs written in C,C++,COBOL,PASCAL, FORTRAN,PL/1 AND ADA. The Precompilers are known as Pro*C,Pro*Cobol,... This form of PL/SQL is known as embedded pl/sql,the language in which pl/sql is embedded is known as the host language. The prcompiler translates the embedded SQL and pl/sql ststements into calls to the precompiler runtime library.The output must be compiled and linked with this library to creater an executable.

22. What is OCI. What are its uses.

Oracle Call Interface is a method of accesing database from a 3GL program. Uses--No precompiler is required,PL/SQL blocks are executed like other DML statements.

                     The OCI library provides

                    -functions to parse SQL statemets

                    -bind input variables

                    -bind output variables

                    -execute statements

                    -fetch the results

23. Difference between database triggers and form triggers.

a) Data base trigger(DBT) fires when a DML operation is performed on a data base table.Form trigger(FT) Fires when user presses a key or navigates between fields on the screen b) Can be row level or statement level No distinction between row level and statement level. c) Can manipulate data stored in Oracle tables via SQL Can manipulate data in Oracle tables as well as variables in forms. d) Can be fired from any session executing the triggering DML statements. Can be fired only from the form that define the trigger. e) Can cause other database triggers to fire.Can cause other database triggers to fire,but not other form triggers.

24. What is an UTL_FILE.What are different procedures and functions associated

with it. UTL_FILE is a package that adds the ability to read and write to operating system files Procedures associated with it are FCLOSE, FCLOSE_ALL and 5 procedures to output data to a file PUT, PUT_LINE, NEW_LINE, PUTF, FFLUSH.PUT, FFLUSH.PUT_LINE,FFLUSH.NEW_LINE. Functions associated with it are FOPEN, ISOPEN.

25. Can you use a commit statement within a database trigger.

26. What is the maximum buffer size that can be specified using the DBMS_OUTPUT.ENABLE function?

1,000,000

Oracle Interview Questions

1. When looking at the estat events report you see that you are getting busy buffer waits. Is this bad? How can you find what is causing it

Buffer busy waits could indicate contention in redo, rollback or data blocks. You need to check the v$waitstat view to see what areas are causing the problem. The value of the "count" column tells where the problem is, the "class" column tells you with what. UNDO is rollback segments, DATA is data base buffers.

2. If you see contention for library caches how can you fix it

Increase the size of the shared pool.

3. If you see statistics that deal with "undo" what are they really talking about

Rollback segments and associated structures.

4. If a tablespace has a default pctincrease of zero what will this cause (in relationship to the smon process)

The SMON process won?t automatically coalesce its free space fragments.

5. If a tablespace shows excessive fragmentation what are some methods to defragment the tablespace? (7.1,7.2 and 7.3 only)

In Oracle 7.0 to 7.2 The use of the 'alter session set events 'immediate trace name coalesce level ts#';? command is the easiest way to defragment contiguous free space fragmentation. The ts# parameter corresponds to the ts# value found in the ts$ SYS table. In version 7.3 the ?alter tablespace coalesce;? is best. If the free space isn?t contiguous then export, drop and import of the tablespace contents may be the only way to reclaim non-contiguous free space.

6. How can you tell if a tablespace has excessive fragmentation

If a select against the dba_free_space table shows that the count of a tablespaces extents is greater than the count of its data files, then it is fragmented.

7. You see the following on a status report: redo log space requests 23 redo log space wait time 0 Is this something to worry about? What if redo log space wait time is high? How can you fix this Since the wait time is zero, no. If the wait time was high it might indicate a need for more or larger redo logs.

8. What can cause a high value for recursive calls? How can this be fixed

A high value for recursive calls is cause by improper cursor usage, excessive dynamic space management actions, and or excessive statement re-parses. You need to determine the cause and correct it By either relinking applications to hold cursors, use proper space management techniques (proper storage and sizing) or ensure repeat queries are placed in packages for proper reuse.

9. If you see a pin hit ratio of less than 0.8 in the estat library cache report is this a problem? If so, how do you fix it

This indicate that the shared pool may be too small. Increase the shared pool size.

10. If you see the value for reloads is high in the estat library cache report is this a matter for concern

Yes, you should strive for zero reloads if possible. If you see excessive reloads then increase the size of the shared pool.

11. You look at the dba_rollback_segs view and see that there is a large number of shrinks and they are of relatively small size, is this a problem? How can it be fixed if it is a problem

A large number of small shrinks indicates a need to increase the size of the rollback segment extents. Ideally you should have no shrinks or a small number of large shrinks. To fix this just increase the size of the extents and adjust optimal accordingly.

12. You look at the dba_rollback_segs view and see that you have a large number of wraps is this a problem

A large number of wraps indicates that your extent size for your rollback segments are probably too small. Increase the size of your extents to reduce the number of wraps. You can look at the average transaction size in the same view to get the information on transaction size.

Oracle Interview Questions

1. You have just started a new instance with a large SGA on a busy existing server. Performance is terrible, what should you check for

The first thing to check with a large SGA is that it isn?t being swapped out.

2. What OS user should be used for the first part of an Oracle installation (on UNIX)

You must use root first.

3. When should the default values for Oracle initialization parameters be used as is

Never

4. How many control files should you have? Where should they be located

At least 2 on separate disk spindles. Be sure they say on separate disks, not just file systems.

5. How many redo logs should you have and how should they be configured for maximum recoverability

You should have at least three groups of two redo logs with the two logs each on a separate disk spindle (mirrored by Oracle). The redo logs should not be on raw devices on UNIX if it can be avoided.

6. You have a simple application with no "hot" tables (i.e. uniform IO and access requirements). How many disks should you have assuming standard layout for SYSTEM, USER, TEMP and ROLLBACK tablespaces

At least 7, see disk configuration answer above.

7. Describe third normal form

Something like: In third normal form all attributes in an entity are related to the primary key and only to the primary key

8. Is the following statement true or false:

"All relational databases must be in third normal form" False. While 3NF is good for logical design most databases, if they have more than just a few tables, will not perform well using full 3NF. Usually some entities will be denormalized in the logical to physical transfer process.

9. What is an ERD

An ERD is an Entity-Relationship-Diagram. It is used to show the entities and relationships for a database logical model.

10. Why are recursive relationships bad? How do you resolve them

A recursive relationship (one where a table relates to itself) is bad when it is a hard relationship (i.e. neither side is a "may" both are "must") as this can result in it not being possible to put in a top or perhaps a bottom of the table (for example in the EMPLOYEE table you couldn?t put in the PRESIDENT of the company because he has no boss, or the junior janitor because he has no subordinates). These type of relationships are usually resolved by adding a small intersection entity.

11. What does a hard one-to-one relationship mean (one where the relationship on both ends is "must")

Expected answer: This means the two entities should probably be made into one entity.

12. How should a many-to-many relationship be handled

By adding an intersection entity table

13. What is an artificial (derived) primary key? When should an artificial (or derived) primary key be used

A derived key comes from a sequence. Usually it is used when a concatenated key becomes too cumbersome to use as a foreign key.

Oracle Interview Questions

1. When should you consider denormalization

Whenever performance analysis indicates it would be beneficial to do so without compromising data integrity.

2. How can you determine if an Oracle instance is up from the operating system level

There are several base Oracle processes that will be running on multi-user operating systems, these will be smon, pmon, dbwr and lgwr. Any answer that has them using their operating system process showing feature to check for these is acceptable. For example, on UNIX a ps -ef|grep dbwr will show what instances are up.

3. Users from the PC clients are getting messages indicating : ORA-06114: (Cnct err, can't get err txt. See Servr Msgs & Codes Manual)

What could the problem be The instance name is probably incorrect in their connection string.

4. Users from the PC clients are getting the following error stack: ERROR: ORA-01034: ORACLE not available ORA-07318: smsget: open error when opening sgadef.dbf file. HP-UX Error: 2: No such file or directory

What is the probable cause The Oracle instance is shutdown that they are trying to access, restart the instance.

5. How can you determine if the SQLNET process is running for SQLNET V1? How about V2

For SQLNET V1 check for the existence of the orasrv process. You can use the command "tcpctl status" to get a full status of the V1 TCPIP server, other protocols have similar command formats. For SQLNET V2 check for the presence of the LISTENER process(s) or you can issue the command "lsnrctl status".

6. What file will give you Oracle instance status information? Where is it located

The alert.ora log. It is located in the directory specified by the background_dump_dest parameter in the v$parameter table.

7. Users aren?t being allowed on the system. The following message is received: ORA-00257 archiver is stuck. Connect internal only, until freed What is the problem The archive destination is probably full, backup the archive logs and remove them and the archiver will re-start.

8. Where would you look to find out if a redo log was corrupted assuming you are using Oracle mirrored redo logs

There is no message that comes to the SQLDBA or SRVMGR programs during startup in this situation, you must check the alert.log file for this information.

9. You attempt to add a datafile and get: ORA-01118: cannot add anymore datafiles: limit of 40 exceeded What is the problem and how can you fix it When the database was created the db_files parameter in the initialization file was set to 40. You can shutdown and reset this to a higher value, up to the value of MAX_DATAFILES as specified at database creation. If the MAX_DATAFILES is set to low, you will have to rebuild the control file to increase it before proceeding.

10. You look at your fragmentation report and see that smon hasn?t coalesced any of you tablespaces, even though you know several have large chunks of contiguous free extents. What is the problem

Check the dba_tablespaces view for the value of pct_increase for the tablespaces. If pct_increase is zero, smon will not coalesce their free space.

11. Your users get the following error: ORA-00055 maximum number of DML locks exceeded What is the problem and how do you fix it The number of DML Locks is set by the initialization parameter DML_LOCKS. If this value is set to low (which it is by default) you will get this error. Increase the value of DML_LOCKS. If you are sure that this is just a temporary problem, you can have them wait and then try again later and the error should clear.

12. You get a call from you backup DBA while you are on vacation. He has corrupted all of the control files while playing with the ALTER DATABASE BACKUP CONTROLFILE command. What do you do

As long as all datafiles are safe and he was successful with the BACKUP controlfile command you can do the following: CONNECT INTERNAL STARTUP MOUNT (Take any read-only tablespaces offline before next step ALTER DATABASE DATAFILE .... OFFLINE;) RECOVER DATABASE USING BACKUP CONTROLFILE ALTER DATABASE OPEN RESETLOGS; (bring read-only tablespaces back online) Shutdown and backup the system, then restart If they have a recent output file from the ALTER DATABASE BACKUP CONTROL FILE TO TRACE; command, they can use that to recover as well. If no backup of the control file is available then the following will be required: CONNECT INTERNAL STARTUP NOMOUNT CREATE CONTROL FILE .....; However, they will need to know all of the datafiles, logfiles, and settings for MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES for the database to use the command.

ORACLE SQL PL/SQL Interview Questions

What is a transaction ?
Answer: A transaction is a set of SQL statements between any two COMMIT and ROLLBACK statements.

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Which of the following is not a schema object : Indexes, tables, public synonyms, triggers and packages ?
Answer: Public synonyms

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What is PL/SQL?
Answer: PL/SQL is Oracle's Procedural Language extension to SQL.The language includes object oriented programming techniques such as encapsulation, function overloading, information hiding (all but inheritance), and so, brings state-of-the-art programming to the Oracle database server and a variety of Oracle tools.

Is there a limit on the size of a PL/SQL block?
Answer: Currently, the maximum parsed/compiled size of a PL/SQL block is 64K and the maximum code size is 100K.You can run the following select statement to query the size of an existing package or procedure. SQL> select * from dba_object_size where name = 'procedure_name'

Before Oracle 7.3 the only means of writing a file was to use DBMS_OUTPUT with the SQL*Plus SPOOL command.
DECLARE
fileHandler UTL_FILE.FILE_TYPE;
BEGIN
fileHandler := UTL_FILE.FOPEN('/home/oracle/tmp', 'myoutput','W');
UTL_FILE.PUTF(fileHandler, 'Value of func1 is %sn', func1(1));
UTL_FILE.FCLOSE(fileHandler);
END;

How can I protect my PL/SQL source code?
Answer: PL/SQL V2.2, available with Oracle7.2, implements a binary wrapper for PL/SQL programs to protect the source code.This is done via a standalone utility that transforms the PL/SQL source code into portable binary object code (somewhat larger than the original).This way you can distribute software without having to worry about exposing your proprietary algorithms and methods.SQL*Plus and SQL*DBA will still understand and know how to execute such scripts.Just be careful, there is no "decode" command available. The syntax is: wrap name=myscript.sql

oname=xxxx.yyy

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Can one use dynamic SQL within PL/SQL? OR Can you use a DDL in a procedure ? How ?
Answer: From PL/SQL V2.1 one can use the DBMS_SQL package to execute dynamic SQL statements.
Eg: CREATE OR REPLACE PROCEDURE DYNSQL AS
cur integer;
rc integer;
BEGIN
cur := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE(cur,'CREATE TABLE X (Y DATE)',

DBMS_SQL.NATIVE);
rc := DBMS_SQL.EXECUTE(cur);
DBMS_SQL.CLOSE_CURSOR(cur);
END;

What are the various types of Exceptions ?
Answer: User defined and Predefined Exceptions.

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What is the difference between a procedure and a function ?
Answer: Functions return a single variable by value whereas procedures do not return any variable by value.Rather they return multiple variables by passing variables by reference through their OUT parameter.

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Can 2 functions have same name & input parameters but differ only by return datatype
Answer: No.

What are the constructs of a procedure, function or a package ?
Answer: The constructs of a procedure, function or a package are :

variables and constants
cursors
exceptions

Why Create or Replace and not Drop and recreate procedures ?
Answer: So that Grants are not dropped.

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Can you pass parameters in packages ? How ?
Answer: Yes.You can pass parameters to procedures or functions in a package.

A triggering event or statement
A trigger restriction
A trigger action

What are the various types of database triggers ?
Answer: There are 12 types of triggers, they are combination of :

Insert, Delete and Update Triggers.
Before and After Triggers.
Row and Statement Triggers.

What is the advantage of a stored procedure over a database trigger ?
Answer: We have control over the firing of a stored procedure but we have no control over the firing of a trigger.

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What are the values of :new and :old in Insert/Delete/Update Triggers ?
Answer: INSERT : new = new value, old = NULL
DELETE : new = NULL, old = old value
UPDATE : new = new value, old = old value

What are cascading triggers? What is the maximum no of cascading triggers at a time?
Answer: When a statement in a trigger body causes another trigger to be fired, the triggers are said to be cascading.Max = 32.

Describe Oracle database's physical and logical structure ?
Answer:

Physical : Data files, Redo Log files, Control file.
Logical : Tables, Views, Tablespaces, etc.

Can you increase the size of a tablespace ? How ?
Answer: Yes, by adding datafiles to it.

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Can you increase the size of datafiles ? How ?
Answer: No (for Oracle 7.0)
Yes (for Oracle 7.3 by using the Resize clause )

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Can Check constraint be used for self referential integrity ? How ?
Answer: Yes.In the CHECK condition for a column of a table, we can reference some other column of the same table and thus enforce self referential integrity.

ONLINE
OFFLINE
PARTLY AVAILABLE
NEEDS RECOVERY
INVALID.

What is the difference between unique key and primary key ?
Answer: Unique key can be null; Primary key cannot be null.

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An insert statement followed by a create table statement followed by rollback ? Will the rows be inserted ?
Answer: No.

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Can you pass a parameter to a cursor ?
Answer: Explicit cursors can take parameters, as the example below shows.A cursor parameter can appear in a query wherever a constant can appear.

CURSOR c1 (median IN NUMBER) IS
SELECT job, ename FROM emp WHERE sal > median;

What are the various types of RollBack Segments ?
Answer: The types of Rollback sagments are as follows :

Public Available to all instances
Private Available to specific instance

Can you use %RowCount as a parameter to a cursor ?
Answer: Yes

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Is the query below allowed :
Select sal, ename Into x From emp Where ename = 'KING' (Where x is a record of Number(4) and Char(15))
Answer: Yes

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Is the assignment given below allowed :
ABC = PQR (Where ABC and PQR are records)
Answer: Yes

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How many rows will the following SQL return : Select * from emp Where rownum < 10;
Answer: 9 rows

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How many rows will the following SQL return : Select * from emp Where rownum = 10;
Answer: No rows

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If all the values from a cursor have been fetched and another fetch is issued, the output will be : error, last record or first record ?
Answer: Last Record

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A table has the following data : [[5, Null, 10]].What will the average function return ?
Answer: 7.5

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Consider a sequence whose currval is 1 and gets incremented by 1 by using the nextval reference we get the next number 2.Suppose at this point we issue an rollback and again issue a nextval.What will the output be ?
Answer: 3

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Definition of relational DataBase by Dr.Codd (IBM)?
Answer: A Relational Database is a database where all data visible to the user is organized strictly as tables of data values and where all database operations work on these tables.

RDBMS - R system
Hierarchical - IMS
N/W - DBTG

Difference between Oracle 6 and Oracle 7
Answer:

ORACLE 7                                 ORACLE 6
Cost based optimizer                   Rule based optimizer
Shared SQL Area                       SQL area allocated for each user
Multi Threaded Server                  Single Threaded Server
Hash Clusters                             Only B-Tree indexing
Roll back Size                            Adjustment No provision
Truncate command                      No provision
Distributed Database                   Distributed Query
Table replication & snapshots       No provision
Client/Server Tech                       No provision

What is Functional Dependency?
Answer: Given a relation R, attribute Y of R is functionally dependent on attribute X of R if and only if each X-value has associated with it precisely one -Y value in R

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While designing in client/server what are the 2 imp.things to be considered ?
Answer: Network Overhead (traffic), Speed and Load of client server

Cannot drop a field
Cannot rename a field
Cannot manage memory
Procedural Language option not provided
Index on view or index on index not provided
View updation problem

When to create indexes ?
Answer: To be created when table is queried for less than 2% or 4% to 25% of the table rows.

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How can you avoid indexes ?
Answer: To make index access path unavailable Use FULL hint to optimizer for full table scan Use INDEX or AND-EQUAL hint to optimizer to use one index or set to indexes instead of another. Use an expression in the Where Clause of the SQL.

What is the result of the following SQL : Select 1 from dual UNION Select 'A' from dual;
Answer: Error

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Can database trigger written on synonym of a table and if it can be then what would be the effect if original table is accessed.
Answer: Yes, database trigger would fire.

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What is the difference between a view and a synonym ?
Answer: Synonym is just a second name of table used for multiple link of database.View can be created with many tables, and with virtual columns and with conditions.But synonym can be on view.

What's the length of SQL integer ?
Answer: 32 bit length

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What is the difference between foreign key and reference key ?
Answer: Foreign key is the key i.e.attribute which refers to another table primary key. Reference key is the primary key of table referred by another table.

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For which relational operators in where clause, index is not used ?
Answer: <> , like '%...' is NOT functions, field +constant, field||''

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Assume that there are multiple databases running on one machine.How can you switch from one to another ?
Answer: Changing the ORACLE_SID

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What are the advantages of Oracle ?
Answer: Portability : Oracle is ported to more platforms than any of its competitors, running on more than 100 hardware platforms and 20 networking protocols. Market Presence : Oracle is by far the largest RDBMS vendor and spends more on R & D than most of its competitors earn in total revenue.This market clout means that you are unlikely to be left in the lurch by Oracle and there are always lots of third party interfaces available. Backup and Recovery : Oracle provides industrial strength support for on-line backup and recovery and good software fault tolerence to disk failure.You can also do point-in-time recovery. Performance : Speed of a 'tuned' Oracle Database and application is quite good, even with large databases.Oracle can manage > 100GB databases. Multiple database support : Oracle has a superior ability to manage multiple databases within the same transaction using a two-phase commit protocol.

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What is a forward declaration ? What is its use ?
Answer: PL/SQL requires that you declare an identifier before using it.Therefore, you must declare a subprogram before calling it.This declaration at the start of a subprogram is called forward declaration.A forward declaration consists of a subprogram specification terminated by a semicolon.

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What are actual and formal parameters ?
Answer: Actual Parameters : Subprograms pass information using parameters.The variables or expressions referenced in the parameter list of a subprogram call are actual parameters.For example, the following procedure call lists two actual parameters named emp_num and amount:
Eg.raise_salary(emp_num, amount);Formal Parameters : The variables declared in a subprogram specification and referenced in the subprogram body are formal parameters.For example, the following procedure declares two formal parameters named emp_id and increase:
Eg.PROCEDURE raise_salary (emp_id INTEGER, increase REAL) IS current_salary REAL;

What are the types of Notation ?

Answer: Position, Named, Mixed and Restrictions.

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If I have an execute privilege on a procedure in another users schema, can I execute his procedure even though I do not have privileges on the tables within the procedure ?
Answer: Yes

Equijoins
Non-equijoins
self join
outer join

What is a package cursor ?
Answer: A package cursor is a cursor which you declare in the package specification without an SQL statement.The SQL statement for the cursor is attached dynamically at runtime from calling procedures.

If you insert a row in a table, then create another table and then say Rollback.In this case will the row be inserted ?
Answer: Yes.Because Create table is a DDL which commits automatically as soon as it is executed.The DDL commits the transaction even if the create statement fails internally (eg table already exists error) and not syntactically.

Yasser

To know the name of database:

select * from global_name;

select name from v$database;

To know the version of database:

SELECT version

         FROM v$instance;

To check the privilege of current user:

select * from session_privs;

To unlock user:

ALTER USER user_name ACCOUNT UNLOCK;

Operators:

SQL> select id from test1;

ID

---------

6 rows selected.

SQL> select id from test2;

ID

---------

SQL> select id from test1

  2  union

  3  select id from test2;

ID

---------

select id from test1

 union all

 select id from test2

ID

---------

select id from test1

intersect

select id from test2

ID

---------

select id from test1

minus

select id from test2

ID

---------

Pages

Tuesday, June 9, 2015

Oracle Interview Question

ORACLE_BASE - The home directory for the Oracle software (e.g. /u01/app/oracle/product/10.2.1) with subdirectories like:

bin – rdbms – sqlplus - etc.

ORACLE_HOME - Below ORACLE_BASE, the files for each database: Oracle Interview Questions

Joins

Join Conditions

Equijoins

Self Joins

Cartesian Products

Inner Joins

Outer Joins

Antijoins

Semijoins

Oracle Interview Questions

Oracle Interview Questions

Oracle Interview Questions

Oracle Interview Questions

ORACLE SQL PL/SQL Interview Questions

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Interesting Links

Pages

Tuesday, June 9, 2015

Oracle Interview Question

ORACLE_BASE - The home directory for the Oracle software (e.g. /u01/app/oracle/product/10.2.1) with subdirectories like: bin – rdbms – sqlplus - etc. ORACLE_HOME - Below ORACLE_BASE, the files for each database: Oracle Interview Questions

Joins

Join Conditions

Equijoins

Self Joins

Cartesian Products

Inner Joins

Outer Joins

Antijoins

Semijoins

Oracle Interview Questions

Oracle Interview Questions

Oracle Interview Questions

Oracle Interview Questions

ORACLE SQL PL/SQL Interview Questions

No comments:

Post a Comment

Interesting Links

ORACLE_BASE - The home directory for the Oracle software (e.g. /u01/app/oracle/product/10.2.1) with subdirectories like:

bin – rdbms – sqlplus - etc.

ORACLE_HOME - Below ORACLE_BASE, the files for each database: Oracle Interview Questions