/* SELECT * FROM wj_user SELECT COUNT(*) FROM wj_user SELECT MIN(id) FROM wj_user SELECT * FROM wj_user ORDER BY LEVEL SELECT * FROM wj_user GROUP BY LEVEL SELECT SUM(id) FROM wj_user GROUP BY LEVEL SELECT COUNT(*) FROM wj_user GROUP BY LEVEL */ SELECT * FROM wj_comment DESC wj_comment ALTER TABLE wj_comment DROP COLUMN id ALTER TABLE wj_comment ADD COLUMN id INT PRIMARY KEY AUTO_INCREMENT SELECT * FROM wj_user u LEFT JOIN wj_comment c ON u.id=c.userId SELECT * FROM wj_user u RIGHT JOIN wj_comment c ON u.id=c.userId SELECT * FROM wj_user u INNER JOIN wj_comment c ON u.id=c.userId SELECT * FROM wj_user u ,wj_comment c