MySQL. Gets all descendants (children) for given ID of parent with depth parameter

descend.sql

DROP PROCEDURE IF EXISTS descend;
CREATE PROCEDURE descend(uid INT, depth INT) BEGIN
  DROP TABLE IF EXISTS descendants;
  DROP TABLE IF EXISTS children;
  CREATE TEMPORARY TABLE descendants (
    id int,
    parent_id int,
    level int
  ) ENGINE = MEMORY;
  INSERT INTO descendants(id, level)
  VALUES (uid, 0);
  CREATE TEMPORARY TABLE children LIKE descendants;
  ALTER TABLE descendants ADD CONSTRAINT UNIQUE(id);

  iter: LOOP
    INSERT INTO children
      SELECT u.id, u.parent_id, level + 1 FROM user u
        JOIN descendants h ON u.parent_id = h.id
      WHERE level < depth;
    DELETE FROM children WHERE id IN (SELECT id FROM descendants);
    IF (SELECT count(*) FROM children) > 0 THEN
      INSERT INTO descendants SELECT * FROM children;
    ELSE
      LEAVE iter;
    END IF;
  END LOOP;
  SELECT * FROM descendants;
#   DROP TABLE descendants;
#   DROP TABLE children;
END;