## 子查询

子查询可以在的位置：
select 
	字段，（子查询）
from
	（子查询内容可以看作一个临时表）
join
	表
on 
	连接内容
where
	（子查询）
	
1、把子查询内容看成一个字段
查询员工表名字对应的部门名称
mysql> select e.ename,(select d.dname from dept d where d.deptno=e.deptno) dname from emp e;
+--------+------------+
| ename  | dname      |
+--------+------------+
| SMITH  | RESEARCH   |
| ALLEN  | SALES      |
| WARD   | SALES      |
| JONES  | RESEARCH   |
| MARTIN | SALES      |
| BLAKE  | SALES      |
| CLARK  | ACCOUNTING |
| SCOTT  | RESEARCH   |
| KING   | ACCOUNTING |
| TURNER | SALES      |
| ADAMS  | RESEARCH   |
| JAMES  | SALES      |
| FORD   | RESEARCH   |
| MILLER | ACCOUNTING |
+--------+------------+
	14 rows in set (0.00 sec)
2、在from后面添加子查询
找出每个部门平均的薪资等级
mysql> select e.*,s.grade from (select e.deptno,avg(e.sal) esal from emp e group by deptno) e join salgrade s on esal between s.losal and s.hisal;
+--------+--------------------+-------+
| deptno | esal               | grade |
+--------+--------------------+-------+
|     10 | 2916.6666666666665 |     4 |
|     20 |               2175 |     4 |
|     30 | 1566.6666666666667 |     3 |
+--------+--------------------+-------+
	3 rows in set (0.00 sec)

mysql> select * from salgrade;
+-------+-------+-------+
| GRADE | LOSAL | HISAL |
+-------+-------+-------+
|     1 |   700 |  1200 |
|     2 |  1201 |  1400 |
|     3 |  1401 |  2000 |
|     4 |  2001 |  3000 |
|     5 |  3001 |  9999 |
+-------+-------+-------+
	5 rows in set (0.00 sec)
3.在where后面添加子查询
找出高于平均薪资的员工信息
mysql> select ename,sal from emp where sal>(select avg(sal) from emp);
+-------+------+
| ename | sal  |
+-------+------+
| JONES | 2975 |
| BLAKE | 2850 |
| CLARK | 2450 |
| SCOTT | 3000 |
| KING  | 5000 |
| FORD  | 3000 |
+-------+------+
	6 rows in set (0.00 sec)
## union 
将查询结果相加（列数要一至）
一、找出工作岗位是salesman和manager的员工
第一种：mysql> select ename,job from emp where job='salesman' or job='manager' order by job;
+--------+----------+
| ename  | job      |
+--------+----------+
| JONES  | MANAGER  |
| BLAKE  | MANAGER  |
| CLARK  | MANAGER  |
| ALLEN  | SALESMAN |
| WARD   | SALESMAN |
| MARTIN | SALESMAN |
| TURNER | SALESMAN |
+--------+----------+
	7 rows in set (0.00 sec)
第二种：mysql> select ename,job from emp where job in('salesman','manager') order by job;
+--------+----------+
| ename  | job      |
+--------+----------+
| JONES  | MANAGER  |
| BLAKE  | MANAGER  |
| CLARK  | MANAGER  |
| ALLEN  | SALESMAN |
| WARD   | SALESMAN |
| MARTIN | SALESMAN |
| TURNER | SALESMAN |
+--------+----------+
	7 rows in set (0.00 sec)
第三种：mysql> select ename,job from emp where job='salesman'
    -> union
    -> select ename,job from emp where job='manager';
+--------+----------+
| ename  | job      |
+--------+----------+
| ALLEN  | SALESMAN |
| WARD   | SALESMAN |
| MARTIN | SALESMAN |
| TURNER | SALESMAN |
| JONES  | MANAGER  |
| BLAKE  | MANAGER  |
| CLARK  | MANAGER  |
+--------+----------+
	7 rows in set (0.00 sec)
有的时候不能用or或in只能用union比如两张不相干的表
例子：mysql> select ename from emp
    -> union
    -> select grade from salgrade;
+--------+
| ename  |
+--------+
| SMITH  |
| ALLEN  |
| WARD   |
| JONES  |
| MARTIN |
| BLAKE  |
| CLARK  |
| SCOTT  |
| KING   |
| TURNER |
| ADAMS  |
| JAMES  |
| FORD   |
| MILLER |
| 1      |
| 2      |
| 3      |
| 4      |
| 5      |
+--------+
	19 rows in set (0.00 sec)
## 表的复制
create table 表名 as select语句
## 将查询结果插入到一张表中
insert into dept1 select *from dept;