【40-40】union

概述

拼接作用

找出工作岗位是SALESMAN和MANAGER的员工

第一种

select ename, job from emp 
where job = 'SALESMAN' or job = 'MANAGER';

第二种

select ename, job from emp 
where job in('SALESMAN', 'MANAGER');

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)

两张不相干的表拼接

mysql> select ename from emp
    -> union
    -> select dname from dept;
+------------+
| ename      |
+------------+
| SMITH      |
| ALLEN      |
| WARD       |
| JONES      |
| MARTIN     |
| BLAKE      |
| CLARK      |
| SCOTT      |
| KING       |
| TURNER     |
| ADAMS      |
| JAMES      |
| FORD       |
| MILLER     |
| ACCOUNTING |
| RESEARCH   |
| SALES      |
| OPERATIONS |
+------------+
18 rows in set (0.00 sec)