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.
An exception handler for the interrupt error SQL_INTERRUPT, SQLSTATE 57014 should only contain non-interruptible statements such as ROLLBACK and ROLLBACK TO SAVEPOINT. If the exception handler contains interruptible statements that are invoked when the connection is interrupted, the database server stops the exception handler at the first interruptible statement and returns the interrupt error.
An exception handler can use the SQLSTATE or SQLCODE special values to determine why a statement failed. Alternatively, the ERRORMSG function can be used without an argument to return the error condition associated with a SQLSTATE. Only the first statement in each WHEN clause can specify this information and the statement cannot be a compound statement.
The demonstration procedures used to illustrate exception handling are based on those used in Default error handling in procedures and triggers.
In this example, additional code handles the column not found error in the InnerProc procedure.
DROP PROCEDURE OuterProc; DROP PROCEDURE InnerProc; 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; CALL OuterProc();
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 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 the above 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.
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