题目:查询小尹老师底下所有的学生
create database test; use test; create table stu( id bigint, classID bigint, name varchar(10) ); create table class( id bigint, teacherID bigint ); create table teacher( id bigint, name varchar(10) ); insert into stu values(1,2,'马哥'); insert into class values(1,2); insert into teacher values(2,'小尹'); update class set id = 1 where id = 2;
错误的 因为左外连接 尽量保存左表的数据 on的条件相当于没有
SELECT S.* FROM `stu` S LEFT JOIN class C on S.classID = C.id LEFT JOIN teacher T on C.teacherID = T.id and T.name = "小尹";
正确的
SELECT S.* FROM `teacher` T LEFT JOIN class C on T.id = C.teacherID LEFT JOIN stu S on S.classID = 2 and T.name = "小尹";
同时备注左外,右外,内连接的区别
SELECT * from users; -- 结果一 SELECT * from t_order; -- 结果二 SELECT u.id, o.* from users u LEFT JOIN t_order o on u.id = o.user_id; -- 结果三 SELECT u.id, o.* from users u RIGHT JOIN t_order o on u.id = o.user_id; -- 结果四 SELECT u.id, o.* from users u INNER JOIN t_order o on u.id = o.user_id; -- 结果五
以上可以看出,左外连接保存左边全部的,右外连接保存右边全部的,内连接是保存双方的交集