mysql的 left join ,right join 和 inner join的区别

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1.left join的使用

SELECT tu.*,tr.* from t_a_user AS tu LEFT JOIN t_a_user_role as tur on tu.id = tur.user_id  LEFT JOIN t_a_role AS tr ON tur.user_id = tr.id;

 

显示所有的用户信息包含某个用户在中间表中没有数据，也就是没有角色的用户也会显示；

2.inner join的使用

SELECT tu.*,tr.* from t_a_user AS tu INNER JOIN t_a_user_role as tur on tu.id = tur.user_id  INNER JOIN t_a_role AS tr ON tur.user_id = tr.id;

显示中间表中的关联信息才会显示，也就是说有个用户必须拥有角色才会显示，没有角色信息是不会显示的；且的关系

3.right join的使用

SELECT tu.*,tr.* from t_a_user AS tu RIGHT JOIN t_a_user_role as tur on tu.id = tur.user_id  RIGHT JOIN t_a_role AS tr ON tur.user_id = tr.id;

显示所有的角色信息包括没有分配给用户的角色信息