Why Is My MySQL Trigger Failing to Update the PTS Column After Manual Execution Succeeds?

Troubleshooting a Non-Functional MySQL Trigger for Updating PTS on Game Statistics

Consider this issue: despite the manual execution of the query succeeding, a trigger intended to automatically update the PTS column based on changed played_games values is not functioning. The trigger syntax is straightforward:

<code class="sql">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</code>

However, further investigation reveals an error when attempting to modify played_games manually: "Can't update table pos_table in stored function/trigger because it's already used by statement which invoked this stored function/trigger."

Explanation:

This error stems from the constraints imposed by stored functions and triggers. They are prohibited from altering tables that the invoking statement has already read from or written to.

Solution:

For triggers triggered on insert operations, if you intend to modify the values being inserted, employ a BEFORE INSERT trigger and adjust the NEW values.

Additionally, the update statement written in the trigger:

<code class="sql">update pos_table set PTS=((NEW.won_games*2)+(NEW.tie_games*1));</code>

is flawed. It updates the entire table rather than a single row. For this scenario, it's more efficient to calculate PTS at display time instead of having a dedicated column. This eliminates the need for a trigger and overcomes the trigger-related issue.

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