How Can I Set a Field Value to Its ID in MySQL Using a Trigger?

MySQL Trigger to Set Field Value to ID

In MySQL, it is not possible to directly update a field to the value of its ID using a trigger. This is because triggers execute before or after a row is inserted, updated, or deleted, but before the auto-increment ID is generated.

To achieve the desired behavior, you can use the following workaround:

1. Create a trigger that checks if the group_id column is null for each inserted row.
2. If null, set the group_id column to the value of the id.
3. Use the BEFORE INSERT trigger type, as it executes before the auto-increment ID is generated.

Here's the MySQL statement for the trigger:

CREATE TRIGGER MyTrigger BEFORE INSERT ON MyTable
FOR EACH ROW
BEGIN
  SET NEW.group_id = COALESCE(NEW.group_id, NEW.id);
END

Unfortunately, this trigger will not work for the desired behavior because in the BEFORE INSERT phase, the auto-generated ID value has not been generated yet. Therefore, if group_id is null, it will default to NEW.id, which is always 0.

Changing the trigger to fire during the AFTER INSERT phase will not work either, as you cannot modify column values after a row has been inserted.

The only solution is to perform the insert and then immediately update the group_id value if it is not set:

INSERT INTO MyTable (group_id, value) VALUES (NULL, 'a');
UPDATE MyTable SET group_id = COALESCE(group_id, id) WHERE id = LAST_INSERT_ID();

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