Exception Handling in PL/SQL

Exception Handling in PL/SQL

In any procedural language, it is critical to remember that the programs are a complete and separate entity from the database. Hence, whenever the program requests rows from the database, the programmer must make sure that the request completed successfully.

In relational databases, the database will always pass a special variable called SQLCODE back to any calling program. The SQLCODE returned from reach call from the program to the database is translated by Oracle into a named Boolean variable (See table below).

PL/SQL Exception Variable	Oracle Error	SQLCODE Value
ACCESS_INTO_NULL	ORA-06530	-6530
CASE_NOT_FOUND	ORA-06592	-6592
COLLECTION_IS_NULL	ORA-06531	-6531
CURSOR_ALREADY_OPEN	ORA-06511	-6511
DUP_VAL_ON_INDEX	ORA-00001	-1
INVALID_CURSOR	ORA-01001	-1001
INVALID_NUMBER	ORA-01722	-1722
LOGIN_DENIED	ORA-01017	-1017
NO_DATA_FOUND	ORA-01403	+100
NOT_LOGGED_ON	ORA-01012	-1012
PROGRAM_ERROR	ORA-06501	-6501
ROWTYPE_MISMATCH	ORA-06504	-6504
SELF_IS_NULL	ORA-30625	-30625
STORAGE_ERROR	ORA-06500	-6500
SUBSCRIPT_BEYOND_COUNT	ORA-06533	-6533
SUBSCRIPT_OUTSIDE_LIMIT	ORA-06532	-6532
SYS_INVALID_ROWID	ORA-01410	-1410
TIMEOUT_ON_RESOURCE	ORA-00051	-51
TOO_MANY_ROWS	ORA-01422	-1422
VALUE_ERROR	ORA-06502	-6502
ZERO_DIVIDE	ORA-01476	-1476

For example, if the database returns a SQLCODE=100, the PL/SQL variable NO_DATA_FOUND will be set to TRUE.

Without exception, all PL/SQL programs should be made to abort whenever an unexpected SQLCODE is returned by the Oracle database.

This can have a disastrous effect on the database, especially when the PL/SQL loads data into tables based upon false premises. To prevent this tragedy, Oracle provides a WHEN OTHERS variable, which is set to TRUE if any unexpected SQLCODE is returned from the Oracle database.

For example, consider the following code:

DECLARE
err_num NUMBER;
err_msg VARCHAR2(100);
BEGIN
...
EXCEPTION
...
WHEN OTHERS THEN
err_num := SQLCODE;
err_msg := SUBSTR(SQLERRM, 1, 100);
INSERT INTO errors VALUES (err_num, err_msg);
END;

Here we see that our exception handling has an EXCEPTIONS area testing WHEN OTHERS. If the WHEN OTHERS Boolean variable is TRUE, the PL/SQL code captures the SQLCODE and the associated error message (SQLERRM), and stores these values into a special Oracle errors table.