Why is my MySQL Trigger not working: Is it a Syntax Error or a Stored Function/Trigger Restriction?

Debugging Ineffective MySQL Trigger: Understanding Stored Function/Trigger Restrictions

Encountering an unresponsive MySQL trigger can be frustrating, especially when the query works flawlessly when executed manually. In such scenarios, the issue often lies within the trigger's syntax or the limitations imposed by stored functions and triggers.

Trigger Syntax Error

The provided trigger syntax seems straightforward and follows the expected pattern. However, it's important to double-check the placement of parentheses and semicolons, ensuring they are correctly placed and not causing any syntactic errors.

Stored Function/Trigger Restrictions

One of the key restrictions to be aware of when using stored functions and triggers is their inability to modify tables that are currently being used by the invoking statement. This restriction is clearly outlined in the documentation:

<code class="text">"A stored function or trigger cannot modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger."</code>

In the provided trigger, the statement update pos_table set PTS=((NEW.won_games*2) (NEW.tie_games*1)); attempts to update the same table (pos_table) that is being modified by the trigger (as it triggers on UPDATE on pos_table). This violation of the aforementioned restriction results in the error encountered when manually editing the played_games column.

Alternative Approach

To bypass the limitation imposed by stored function/trigger restrictions, it is recommended to consider using a different trigger type, such as BEFORE INSERT, if you intend to modify values during insertion. However, in this particular case, the calculation of PTS can be performed directly when displaying the values, eliminating the need for a stored trigger or function. This approach simplifies the code and avoids potential conflicts with stored function/trigger restrictions.

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