MySQL trigger to update a field to the value of id

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 ;

This works for me

DELIMITER $$
CREATE TRIGGER `myTriggerNameHere`
BEFORE INSERT ON `table` FOR EACH ROW
BEGIN
    SET NEW.group_id = IF(NEW.group_id IS NULL, LAST_INSERT_ID()+1, NEW.group_id);
END;
$$
DELIMITER ;

I don't know of any way to do this in one statement, even using a trigger.

The trigger solution that @Lucky suggested would look like this in MySQL:

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

However, there's a problem. 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.

But if you change this trigger to fire during the AFTER INSERT phase, so you have access to the generated value of NEW.id, you can't modify column values.

MySQL doesn't support expressions for the DEFAULT of a column, so you can't declare this behavior in the table definition either. *Update: MySQL 8.0.13 supports DEFAULT (<expression>) but the expression still can't depend on an auto-increment value (this is documented).

The only solution is to do the INSERT, and then immediately do an UPDATE to change the group_id if it's 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();