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.
Some exceptions (see the following table) have been pre-defined by Oracle in the STANDARD package. You can also declare your own exceptions as follows:
DECLARE exception_name EXCEPTION;
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:
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.
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:
where exception_name is the name of an exception that you have declared, or that is declared in the STANDARD package.
If you use the RAISE statement inside an exception handler, you can leave off an exception name to re-raise the current exception:
This syntax is not valid outside the exception section.
The RAISE_APPLICATION_ERROR built-in 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.
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.
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:
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;
EXCEPTION WHEN OTHERS THEN ...
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.
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.
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 hooked,' ||'SQLcode='||SQLCODE); DBMS_OUTPUT.PUT_LINE(SQLERRM); END IF; END;
Gives the output of:
widget_count is too small
The DBMS_UTILITY.FORMAT_ERROR_STACK and DBMS_UTILITY.FORMAT_CALL_STACK procedures can be used to capture the full error stack and call stack. See the O'Reilly & Associates book Oracle PL/SQL Built-in Packages for more information on DBMS_UTILITY.
When an exception is raised in a PL/SQL block, it does not rollback 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 due to 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.
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