It is often desirable to intercept certain types of errors and handle them within a procedure or trigger, rather than pass the error back to the calling environment. This is done through the use of an exception handler.
You define an exception handler with the EXCEPTION part of a compound statement (see Using compound statements). Whenever an error occurs in the compound statement, the exception handler executes. Unlike errors, warnings do not cause exception handling code to be executed. Exception handling code also executes if an error appears in a nested compound statement or in a procedure or trigger invoked anywhere within the compound statement.
Remember to drop both the InnerProc and OuterProc procedures by entering the following commands in the SQL Statements pane before continuing with the tutorial:
DROP PROCEDURE OuterProc; DROP PROCEDURE InnerProc;
The demonstration procedures used to illustrate exception handling are based on those used in Default error handling in procedures and triggers. In this case, additional code handles
the column not found error in the InnerProc procedure.
CREATE PROCEDURE OuterProc() BEGIN MESSAGE 'Hello from OuterProc.' TO CLIENT; CALL InnerProc(); MESSAGE 'SQLSTATE set to ', SQLSTATE,' in OuterProc.' TO CLIENT END; CREATE PROCEDURE InnerProc() BEGIN DECLARE column_not_found EXCEPTION FOR SQLSTATE '52003'; MESSAGE 'Hello from InnerProc.' TO CLIENT; SIGNAL column_not_found; MESSAGE 'Line following SIGNAL.' TO CLIENT; EXCEPTION WHEN column_not_found THEN MESSAGE 'Column not found handling.' TO CLIENT; WHEN OTHERS THEN RESIGNAL ; END;
The EXCEPTION clause declares the exception handler. The lines following EXCEPTION do not execute unless an error occurs. Each WHEN clause specifies an exception name (declared with a DECLARE statement) and the statement or statements to be executed in the event of that exception. The WHEN OTHERS THEN clause specifies the statement(s) to be executed when the exception that occurred does not appear in the preceding WHEN clauses.
In this example, the statement RESIGNAL passes the exception on to a higher-level exception handler. RESIGNAL is the default action if WHEN OTHERS THEN is not specified in an exception handler.
The following statement executes the OuterProc procedure:
The Interactive SQL Messages tab then displays the following:
Hello from OuterProc.
Hello from InnerProc.
Column not found handling.
SQLSTATE set to 00000 in OuterProc.
The EXCEPTION handler executes, rather than the lines following the SIGNAL statement in InnerProc.
As the error encountered was a
column not found error, the MESSAGE statement included to handle the error executes, and SQLSTATE resets to zero (indicating no errors).
After the exception handling code executes, control passes back to OuterProc, which proceeds as if no error was encountered.
You should not use ON EXCEPTION RESUME together with explicit exception handling. The exception handling code is not executed if ON EXCEPTION RESUME is included.
If the error handling code for the
column not found exception is simply a RESIGNAL statement, control passes back to the OuterProc procedure with SQLSTATE still set at the value 52003. This is just as if there were no error handling code in InnerProc. Since there is no error handling code in OuterProc, the procedure fails.
When an exception is handled inside a compound statement, the compound statement completes without an active exception and the changes before the exception are not reversed. This is true even for atomic compound statements. If an error occurs within an atomic compound statement and is explicitly handled, some but not all of the statements in the atomic compound statement are executed.