MySQL trigger to update a field to the value of id

Question

I would like to have a trigger to perform following operation for inserted records:

 # pseudocode
 if new.group_id is null
    set new.group_id = new.id
 els         


        

Answer 1

I am answering here, as in the accepted answer Bill Karwin states:

In the BEFORE INSERT phase, the auto-generated id value hasn't been generated yet. So if group_id is null, it defaults to NEW.id which is always 0.

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I have an answer for it - for OP (and the visitors to come), here are few points: You cannot update the table from where the trigger gets invoked, for it you'll get Error 1442:

Error Code: 1442
Can't update table 'MyTable' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

1.to Update the new row Use BEFORE INSERT ON trigger, this way you can update all the fields for the new row, which can be accessible via NEW operator i.e.

set NEW.group_id = NEW.id

2.Get auto_increment value before insert:

SELECT AUTO_INCREMENT FROM information_schema.TABLES WHERE TABLE_SCHEMA=DATABASE() AND TABLE_NAME='MyTable'

To sum up - the trigger SQL for 'd be something as following:

DELIMITER //
DROP TRIGGER IF EXISTS MyTrigger//
CREATE TRIGGER MyTrigger BEFORE INSERT ON MyTable
FOR EACH ROW BEGIN
    IF new.group_id IS NULL
        set @auto_id := (SELECT AUTO_INCREMENT FROM INFORMATION_SCHEMA.TABLES
                         WHERE TABLE_NAME='MyTable' AND TABLE_SCHEMA=DATABASE() ); 
        set NEW.group_id = @auto_id;
    ENF IF;
END;
//
DELIMITER ;