左外连接的错误

Posted 2020-12-10 ykpkris

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";