MySQL exception handler access exception being handled

Question

I'm trying to rollback on an error, but still let the client receive the error. This might actually be impossible, unless there is a way to access the error in an exception handler.

It's possible to "throw" from an exception, i.e. it's possible to raise a signal:

CREATE PROCEDURE p ()
BEGIN
  DECLARE EXIT HANDLER FOR SQLEXCEPTION
  BEGIN
    SIGNAL SQLSTATE VALUE '99999'
      SET MESSAGE_TEXT = 'An error occurred';
  END;
  DROP TABLE no_such_table;
END;

But this sample code from the MySQL doc looks horrible, because it literally swallows all errors and jams them into one.

SHOW ERRORS seems relevant, but I don't see any way to work with it programmatically, e.g. SELECT Code FROM (SHOW ERRORS); is not possible.

Is this possible? Is there a better practice that I'm missing entirely?

Answer 1

Looks like RESIGNAL is what you are looking for.

RESIGNAL makes it possible to both handle an error and return the error information. Otherwise, by executing an SQL statement within the handler, information that caused the handler's activation is destroyed. RESIGNAL also can make some procedures shorter if a given handler can handle part of a situation, then pass the condition “up the line” to another handler.

DELIMITER $$

DROP PROCEDURE IF EXISTS `test`.`resig` $$
CREATE PROCEDURE `test`.`resig` ()
BEGIN

DECLARE EXIT HANDLER FOR SQLEXCEPTION
BEGIN
  SELECT 'I executed something before throwing the error' as `this_works`;
  RESIGNAL;
END;

SELECT foo FROM bar WHERE baz = 0;

END $$

DELIMITER ;


mysql> call resig();
+------------------------------------------------+
| this_works                                     |
+------------------------------------------------+
| I executed something before throwing the error |
+------------------------------------------------+
1 row in set (0.00 sec)

ERROR 1054 (42S22): Unknown column 'foo' in 'field list'

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