Why Can't I Update a Table Inside a MySQL Stored Function or Trigger?

MySQL Error: Updates Prohibited Within Stored Functions/Triggers

When attempting to update a table within a MySQL stored function or trigger, 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.

Understanding the Error

This error stems from the restriction in MySQL that prohibits modifying a table while an INSERT trigger is executing. Such modifications could potentially lead to deadlocks or infinite recursive loops.

Avoiding the Error

To avoid this error, consider the following approaches:

• Use NEW and OLD Fields:

  • Access new values within the trigger using NEW.fieldname syntax.
  • Access old values within the trigger using OLD.fieldname syntax (if performing an UPDATE).

• Modify the Trigger Context:

  • Change the trigger type from AFTER INSERT to BEFORE INSERT.
  • Move table modification code outside the stored function or trigger.

Example Using BEFORE INSERT Trigger:

Suppose you have a full_brand_name field and need to create a short_name field containing the first two letters in uppercase:

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

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