Why Can't I Update a Table Inside a Trigger That's Modifying the Same Table in MySQL?

Troubleshooting Triggers: Incompatibility with Table Modification

In MySQL, triggers are a powerful tool used to automate database actions based on specific events. However, when a trigger encounters issues in execution, troubleshooting is crucial. This article addresses a common error faced when a MySQL trigger fails to work.

Issue Description:

A user reported that their trigger was not updating a specific column (PTS) when the corresponding played_games column in the pos_table changed. Despite the query working manually, the trigger failed to trigger an update.

Trigger Structure:

CREATE TRIGGER `upd_PTS` AFTER UPDATE ON `pos_table`
FOR EACH ROW BEGIN
    IF (NEW.played_games  OLD.played_games)
    THEN  
        update pos_table set PTS=((NEW.won_games*2)+(NEW.tie_games*1));
    END IF;
END

Error Encountered:

When attempting to manually edit the played_games column, the user received an error stating: "Can't update table pos_table in stored function/trigger because it's already used by statement which invoked this stored function/trigger".

Resolution:

The issue stemmed from the fact that the trigger was attempting to modify the same table (pos_table) that was being referenced in the triggering event (update of played_games). As documented in MySQL's Stored Program Restrictions, a stored function or trigger cannot modify a table that is already being used by the invoking statement.

Alternative Solution:

To avoid this issue, it is recommended to use a BEFORE INSERT trigger instead. In this case, the trigger could be modified as follows:

CREATE TRIGGER `upd_PTS` BEFORE INSERT ON `pos_table`
FOR EACH ROW BEGIN
    SET NEW.PTS = ((NEW.won_games*2)+(NEW.tie_games*1));
END

Additional Considerations:

In addition to the error encountered, the article also highlights the issue with storing a calculated column like PTS. Since it can be easily calculated from other columns during display, it may be more efficient and less prone to errors to avoid storing it as a separate column.

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