How to do it in mysql: If id=idold THEN UPDATE status=1

Answer 1

UPDATE users u
SET status = 1
WHERE EXISTS (SELECT id FROM usersold WHERE id = u.id)

Alternate version:

UPDATE users
SET status = 1
WHERE id IN (SELECT id FROM usersold)

You should test and, depending on your database, you may find one performs better than the other although I expect any decent database will optimize then to be much the same anyway.