Question

What does the MySQL USING statement do and is there any docs on this?

MySQL USING statement example

DELETE FROM l, s 
USING l
INNER JOIN s ON s.skill_id = l.id
WHERE s.user_id = 3

Answer 1

In a DELETE, you can list tables after the USING clause, from which rows do not get deleted (i.e., to only be part of the WHERE clause). For example:

DELETE FROM t1, t2 
  USING t1 
    INNER JOIN t2 
    INNER JOIN t3
  WHERE t1.id=t2.id AND 
        t2.id=t3.id;

Your particular example can be achieved without USING in this fashion:

DELETE l,s 
FROM l 
  INNER JOIN s 
    ON s.skill_id = l.id 
WHERE s.user_id = 3