标签:查看 where 内连接 组合 右外连接 技术 nbsp image 技术分享
多表查询,SQL JOINS
1. inner join内连接
select s.name as student_name,t.name as teacher_name from students as s inner join teachers as t on s.teacherid=t.tid; /*2张表取交集,内连接*/
2. 交叉连接
select * from students cross join teachers; /*笛卡尔乘积,students表内容和teacher表内容各自组合一遍*/
3. 左外连接
select s.name as student_name,t.name as teacher_name from students as s left outer join teachers as t on s.teacherid=t.tid;
1.)左内连接
select s.name as student_name,t.name as teacher_name from students as s left outer join teachers as t on s.teacherid=t.tid and t.name is null;
4.右外连接
select s.name as student_name,t.name as teacher_name from teachers as t right outer join students as s on s.teacherid=t.tid;
5.完全外连接
union
MariaDB [hellodb]> select s.name as student_name,t.name as teacher_name from students as s left outer join teachers as t on s.teacherid=t.tid
-> union
-> select s.name as student_name,t.name as teacher_name from students as s right outer join teachers as t on s.teacherid=t.tid;
5.自连接
select s1.name as emp, s2.name as leader from students as s1 inner join students as s2 on s1.teacherid=s2.stuid; /*使用内连接取2张表的交集*/
select s1.name as emp, s2.name as leader from students as s1 left outer join students as s2 on s1.teacherid=s2.stuid; /*使用左外连接取2张表的交集*/
6.子查看
#查看分数大于平均分的分数
select stuid,score from scores where score > (select avg(score) from scores);
#查询分数大于平均分数,学生的姓名和分数
select st.name,sc.score from students as st inner join scores as sc on st.stuid=sc.stuid and score > (select avg(score) from scores);
标签:查看 where 内连接 组合 右外连接 技术 nbsp image 技术分享
原文地址:https://www.cnblogs.com/ifelz/p/9159826.html