MySQL子查询详细教程

select 
	....(select)..
from 
	....(select)..
where 
	....(select)..

select * from emp where sal > (select avg(sal) from emp) ; 

SELECT d.dname,AVG(s.grade) '部门平均薪资' 
FROM emp e
JOIN salgrade s ON e.sal BETWEEN s.losal AND s.hisal 
JOIN dept d ON e.deptno = d.deptno
GROUP BY d.dname

写法一：看作临时表
SELECT t.ename,d.dname
FROM dept d JOIN (SELECT e.ename,e.deptno FROM emp e) t ON t.deptno = d.deptno   

写法二：连接查询
SELECT e.ename,d.dname FROM emp e JOIN dept d ON e.deptno = d.deptno

写法三：select后嵌套子查询
SELECT e.ename,(SELECT d.dname FROM dept d WHERE d.deptno = e.deptno) AS dept FROM emp e ;

SELECT ename,job FROM emp e WHERE job = 'CLERK'
UNION
SELECT ename,job FROM emp e WHERE job = 'SALESMAN'

SELECT ename FROM emp 
UNION
SELECT dname FROM dept