Why Does MySQL Block Table Modification in Stored Functions and Triggers?

MySQL Stored Function/Trigger Error: Table Modification Blocked

When executing a stored function or trigger in MySQL, you may encounter the error:

Error: Can't update table 'brandnames' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

This error occurs when attempting to update a table from within a stored function or trigger, which is prohibited due to potential deadlocks or infinite recursive calls.

The INSERT trigger, such as the one provided:

CREATE TRIGGER `capital` AFTER INSERT ON `brandnames`
FOR EACH ROW UPDATE brandnames
SET bname = CONCAT( UCASE( LEFT( bname, 1 ) ) , LCASE( SUBSTRING( bname, 2 ) ) )

attempts to update the brandnames table while the INSERT operation is still active. MySQL prevents this to maintain data integrity and avoid potential errors.

To resolve this issue, consider using the following methods:

• Accessing New Values: Use NEW.fieldname to access the values set during the INSERT operation. This allows you to modify derived values without affecting the table.
• Using Before Triggers: If possible, move the table modification logic to a BEFORE INSERT trigger. This allows you to set derived values before the INSERT completes, avoiding the error.

For example, the following trigger will update the small_name column based on values in the full_name column:

CREATE TRIGGER `capital` BEFORE INSERT ON `brandnames`
FOR EACH ROW BEGIN
  SET NEW.small_name = CONCAT(UCASE(LEFT(NEW.full_name,1)) , LCASE(SUBSTRING(NEW.full_name,2)))
END

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