多表合併
1.多表縱向合併union
MariaDB [hellodb]> select from teachers union select from students;
2.交叉連接
select from students cross join select from teachers;
3.內連接inner join
MariaDB [hellodb]> select * from students inner join teachers on students.teacherid=teachers.tid; 
MariaDB [hellodb]> select stuid,s.name,tid,t.name from students s inner join teachers t on s.teacherid=t.tid; 
MariaDB [hellodb]> select stuid,s.name,tid,t.name from students s,teachers t where s.teacherid=t.tid; 
1）內連接後過濾數據
select stuid,s.name,tid,t.name from students s inner join teachers t on s.teacherid=t.tid and stuid >3; 
4.左外連接
MariaDB [hellodb]> select stuid,s.name,tid,t.name from students as s left outer join teachers as t on s.teacherid=t.tid;
5.右外連接
MariaDB [hellodb]> select stuid,s.name,tid,t.name from students as s right join teachers as t on s.teacherid=t.tid; 
6.左外連接擴展用法
MariaDB [hellodb]> select stuid,s.name,teacherid,tid,t.name from students as s left outer join teachers as t on s.teacherid=t.tid where t.tid is null; 
7.完全外連接 full outer joion
select from students left join teachers on students.teacherid=teachers.tid
union
select from students right join teachers on students.teacherid=teachers.tid;
子查詢：select 的執行結果，被其它SQL調用
select * from students where age < ( select avg(age) from students );
MariaDB [hellodb]> select stuid,name,age from students where age > (select avg(age) from students); 
8.完全外連接的擴展示例
MariaDB [hellodb]> select * from (select s.stuid,s.name s_name,s.teacherid,t.tid,t.name t_name from students s left join teachers t on s.teacherid=t.tid union select s.stuid,s.name s_name,s.teacherid,t.tid,t.name t_name from students s right join teachers t on s.teacherid=t.tid) as a where a.teacherid is null or a.tid is null; 
自連接
MariaDB [hellodb]> select emp.name emp_name , leader.name leader_name from employee emp inner join employee as leader on emp.leaderid=leader.id; 
三張表連接示例
MariaDB [hellodb]> select st.stuid,st.name,sc.score,co.course from students as st inner join scores sc on st.stuid=sc.stuid inner join courses co on sc.courseid=co.courseid; 