MySQL: re-use auto-increment during insert

Question

I\'m designing a comments MySQL db, and my comments table has fields:

• id primary key, auto-incr
• thread int, not null

Answer 1

Do I have to use 2 queries?

Yes. As you discovered, the id value hasn't been generated yet in a BEFORE INSERT trigger. But you can't change your NEW.thread value in an AFTER INSERT trigger.

You can't rely on reading the INFORMATION_SCHEMA, because you can cause a race condition.

You'll just have to do the INSERT, and then immediately execute:

UPDATE comments SET thread=id WHERE id=LAST_INSERT_ID() AND thread IS NULL;

If it's a root comment.

See also my past answers on the similar topic:

• Concatenating a string and primary key Id while inserting
• Two autoincrements columns or autoincrement and same value in other column