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

MySQL Error: Can't Update Table in Stored Function/Trigger

Question:

When attempting to insert a new row into a MySQL table using a trigger, the following error occurs:

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

Answer:

This error is encountered when the INSERT trigger attempts to modify the table that invoked it. MySQL prohibits this behavior for two main reasons:

• Locking Conflicts: The INSERT might perform locking operations, which can lead to deadlocks if the trigger also attempts to lock the table.
• Infinite Recursion: If the trigger updates a row in the same table, it can cause the same trigger to be fired indefinitely, resulting in an infinite loop.

To resolve this error, consider using the following approaches:

• Accessing New Values: Instead of modifying the table in the trigger, use NEW.fieldname to access the newly inserted values and set other fields accordingly.
• Pre-Insertion Modification: Use a BEFORE INSERT trigger to modify the values before they are inserted into the table. This avoids the infinite recursion issue.

For example, to create a short name field from the first two characters of a full name, the following BEFORE INSERT trigger can be used:

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

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