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
 else
    # don't touch it
 end

More clearly: say I have one table with three columns: id primary key, group_id int, value varchar.

When I insert with group_id like that:

INSERT INTO table(value, group_id) VALUES ('a', 10)

I'd like to have:

id | group_id | value
---+----------+------
 1 |       10 | a

but when I omit group_id:

INSERT INTO table(value) VALUES ('b')

it should be automatically set to the id of this record:

id | group_id | value
---+----------+------
 2 |        2 | b

Is it possible with a trigger? (I know I can update the record after inserting but having the trigger would be nicer.)

Answer 1

I believe that this will work for you

I have two tables

test_b: a_id, value
test_c: a_id, value

And here is a trigger on the insert of test b. It checks to see if a_id is null and if it is it inserts 0

CREATE TRIGGER test_b AFTER INSERT ON test_b
  FOR EACH ROW 
    INSERT INTO test_c (a_id, value) VALUES (IFNULL(NEW.a_id, 0),NEW.value)

Answer 2

This trigger should do what you asked.

   CREATE TRIGGER mytrigger BEFORE INSERT ON mytable
          IF new.group_id IS NULL
            SET new.group_id = new.id
          END IF
      END;

It's copied from a very similar example in the MYSQL documentation page.

Answer 3

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.

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();