[jira] Commented: (DERBY-172) Trigger activation

["Daniel John Debrunner (JIRA)" <de...@db.apache.org>]

     [ http://issues.apache.org/jira/browse/DERBY-172?page=comments#action_60634 ]
     
Daniel John Debrunner commented on DERBY-172:
---------------------------------------------

BEFORE triggers do work but they have the restriction that the trigger action must be read-only, that is a SELECT statement or a VALUES claues. The action statement can call a user defined Java function that could send e-mail or perform validation.

Enabling the before triggers was last minute item before the code was contributed.

> Trigger activation
> ------------------
>
>          Key: DERBY-172
>          URL: http://issues.apache.org/jira/browse/DERBY-172
>      Project: Derby
>         Type: Bug
>   Components: Documentation
>     Versions: 10.0.2.0
>  Environment: Windows XP SP1 Windows
>     Reporter: George Baklarz
>     Priority: Minor

>
> I image that this is a documentation problem. The "words" around triggers seem to imply the ability to do actions "BEFORE" the INSERT/UPDATE/DELETE. For instance:
> "Along with constraints, triggers can help enforce data integrity rules with actions such as cascading deletes or updates. Triggers can also perform a variety of functions such as issuing alerts, updating other tables, sending e-mail, and other useful actions."
> Well, the CREATE TRIGGER command only allows AFTER as an option (even though the SYSTRIGGERS catalog uses "B" for before and "A" for after) and it won't allow a CALL statement to be processed. Seems to me that there aren't any ways to send an e-mail, issue alerts (no SIGNAL function either), or enforce integrity rules. How can you enforce integrity rules if invalid data comes in? I can't force a SIGNAL or error with the trigger, so this is useless for validating changes to the data.
> In addition, there appears to be some logic regarding BEFORE triggers in Derby, but its not currently working.
> CREATE TRIGGER CB NO CASCADE BEFORE
>    INSERT ON BONUSES
>       REFERENCING NEW AS n
>       FOR EACH ROW MODE DB2SQL
>       UPDATE BONUSES
>          SET BONUS =
>              CASE
>                 WHEN (n.BONUS = 0) THEN 100
>                 ELSE n.BONUS
>              END;
> ERROR 42Z9D: 'UPDATE' statements are not allowed in 'BEFORE' triggers.

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