1.10 Exception Handling | Oracle PL/SQL Language Pocket Reference (Pocket Reference (OReilly))

PL/SQL allows developers to raise and handle errors (exceptions) in a very flexible and powerful way. Each PL/SQL block can have its own exception section in which exceptions can be trapped and handled (resolved or passed on to the enclosing block).

When an exception occurs (is raised) in a PL/SQL block, its execution section immediately terminates. Control is passed to the exception section.

Every exception in PL/SQL has an error number and error message; some exceptions also have names.

1.10.1 Declaring Exceptions

Some exceptions (see the following table) have been pre-defined by Oracle in the STANDARD package or other built-in packages, such as UTL_FILE. You can also declare your own exceptions as follows:

DECLARE    exception_name EXCEPTION;

Error	Named exception
ORA-00001	DUP_VAL_ON_INDEX
ORA-00051	TIMEOUT_ON_RESOURCE
ORA-00061	TRANSACTION_BACKED_OUT
ORA-01001	INVALID_CURSOR
ORA-01012	NOT_LOGGED_ON
ORA-01017	LOGIN_DENIED
ORA-01403	NO_DATA_FOUND
ORA-01410	SYS_INVALID_ROWID
ORA-01422	TOO_MANY_ROWS
ORA-01476	ZERO_DIVIDE
ORA-01725	USERENV_COMMMITSCN_ERROR
ORA-01722	INVALID_NUMBER
ORA-06500	STORAGE_ERROR
ORA-06501	PROGRAM_ERROR
ORA-06502	VALUE_ERROR
ORA-06504	ROWTYPE_MISMATCH
ORA-06511	CURSOR_ALREADY_OPEN
ORA-06530	ACCESS_INTO_NULL
ORA-06531	COLLECTION_IS_NULL
ORA-06532	SUBSCRIPT_OUTSIDE_LIMIT
ORA-06533	SUBSCRIPT_BEYOND_COUNT
ORA-09592	CASE_NOT_FOUND
ORA-30625	SELF_IS_NULL
ORA-29280	INVALID_PATH
ORA-29281	INVALID_MODE
ORA-29282	INVALID_FILEHANDLE
ORA-29283	INVALID_OPERATION
ORA-29284	READ_ERROR
ORA-29285	WRITE_ERROR
ORA-29286	INTERNAL_ERROR
ORA-29287	INVALID_MAXLINESIZE
ORA-29288	INVALID_FILENAME
ORA-29289	ACCESS_DENIED
ORA-29290	INVALID_OFFSET
ORA-29291	DELETE_FAILED
ORA-29292	RENAME_FAILED

An exception can be declared only once in a block, but nested blocks can declare an exception with the same name as an outer block. If this multiple declaration occurs, scope takes precedence over name when handling the exception. The inner block's declaration takes precedence over a global declaration.

When you declare your own exception, you must RAISE it explicitly. All declared exceptions have an error code of 1 and the error message "User-defined exception," unless you use the EXCEPTION_INIT pragma.

You can associate an error number with a declared exception with the PRAGMA EXCEPTION_INIT statement using the following syntax:

DECLARE    exception_name EXCEPTION;    PRAGMA EXCEPTION_INIT (exception_name,        error_number);

where error_number is a literal value (variable references are not allowed). This number can be an Oracle error, such as -1855, or an error in the user-definable -20000 to -20999 range.

1.10.2 Raising Exceptions

An exception can be raised in three ways:

By the PL/SQL runtime engine
By an explicit RAISE statement in your code
By a call to the built-in function RAISE_APPLICATION_ERROR

The syntax for the RAISE statement is:

RAISE exception_name;

where exception_name is the name of an exception that you have declared, or an exception that is declared in the STANDARD package. If you use the RAISE statement inside an exception handler, you can omit the exception name to re-raise the current exception:

RAISE;

This syntax is not valid outside the exception section.

The RAISE_APPLICATION_ERROR built-in function has the following header:

RAISE_APPLICATION_ERROR (    num BINARY_INTEGER,    msg VARCHAR2,    keeperrorstack BOOLEAN DEFAULT FALSE);

where num is the error number (an integer between -20999 and -20000), msg is the associated error message, and keeperrorstack controls the contents of the error stack.

1.10.3 Scope

The scope of an exception section is that portion of the code that is "covered" by the exception section. An exception handler will only handle or attempt to handle exceptions raised in the executable section of the PL/SQL block. Exceptions raised in the declaration or exception sections are automatically passed to the outer block. Any line or set of PL/SQL code can be placed inside its own block and given its own exception section. This allows you to limit the propagation of an exception.

1.10.4 Propagation

Exceptions raised in a PL/SQL block propagate to an outer block if they are unhandled or re-raised in the exception section. When an exception occurs, PL/SQL looks for an exception handler that checks for the exception (or is the WHEN OTHERS clause) in the current block. If a match is not found, then PL/SQL propagates the exception to the enclosing block or calling program. This propagation continues until the exception is handled or propagated out of the outermost block, back to the calling program. In this case, the exception is "unhandled" and (1) stops the calling program, and (2) causes an automatic rollback of any outstanding transactions.

Once an exception is handled, it will not propagate upward. If you want to trap an exception, display a meaningful error message, and have the exception propagate upward as an error, you must re-raise the exception. The RAISE statement can re-raise the current exception or raise a new exception, as shown here:

PROCEDURE delete_dept(deptno_in IN NUMBER) DECLARE    still_have_employees EXCEPTION    PRAGMA EXCEPTION_INIT(still_have_employees.        -2292) BEGIN DELETE FROM dept WHERE deptno = deptno_in; EXCEPTION    WHEN still_have_employees    THEN         DBMS_OUTPUT.PUT_LINE ('Please delete employees in dept first');    ROLLBACK;    RAISE;  /* Re-raise the current exception. */ END;

1.10.4.1 WHEN OTHERS clause

Use the WHEN OTHERS clause in the exception handler as a catch-all to trap any exceptions that are not handled by specific WHEN clauses in the exception section. If present, this clause must be the last exception handler in the exception section. You specify this clause as follows:

EXCEPTION    WHEN OTHERS    THEN       ...

1.10.4.2 SQLCODE and SQLERRM

SQLCODE and SQLERRM are built-in functions that provide the SQL error code and message for the current exception. Use these functions inside the exception section's WHEN OTHERS clause to handle specific errors by number. The EXCEPTION_INIT pragma allows you to handle errors by name. For example, the following code:

CREATE TABLE err_test    (widget_name   VARCHAR2(100)    ,widget_count  NUMBER    ,CONSTRAINT no_small_numbers CHECK        (widget_count > 1000)); BEGIN    INSERT INTO err_test (widget_name, widget_count)    VALUES ('Athena',2); EXCEPTION    WHEN OTHERS THEN    IF SQLCODE = -2290       AND SQLERRM LIKE '%NO_SMALL_NUMBERS%'    THEN       DBMS_OUTPUT.PUT_LINE('widget_count is too          small');    ELSE       DBMS_OUTPUT.PUT_LINE('Exception not handled,'          ||'SQLcode='||SQLCODE);       DBMS_OUTPUT.PUT_LINE(SQLERRM);    END IF; END;

produces this output:

widget_count is too small

The built-in package DBMS_UTILITY's FORMAT_ERROR_STACK and FORMAT_CALL_STACK procedures can be used to capture the full error stack and call stack. See the book Oracle Built-in Packages for more information on DBMS_UTILITY.

1.10.4.3 Exceptions and DML

When an exception is raised in a PL/SQL block, it does not roll back your current transaction, even if the block itself issued an INSERT, UPDATE, or DELETE. You must issue your own ROLLBACK statement if you want to clean up your transaction as a result of the exception.

If your exception goes unhandled (propagates out of the outermost block), however, most host environments will then force an automatic, unqualified rollback of any outstanding changes in your session.