mysql中union的用法[转载]

https://blog.csdn.net/qq_45148387/article/details/116357408?utm_medium=distribute.pc_relevant.none-task-blog-2~default~baidujs_baidulandingword~default-0-116357408-blog-123897432.pc_relevant_3mothn_strategy_recovery&spm=1001.2101.3001.4242.1&utm_relevant_index=3

union（可以将查询结果集相加）

案例：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';
+--------+----------+
| ename  | job      |
+--------+----------+
| ALLEN  | SALESMAN |
| WARD   | SALESMAN |
| JONES  | MANAGER  |
| MARTIN | SALESMAN |
| BLAKE  | MANAGER  |
| CLARK  | MANAGER  |
| TURNER | SALESMAN |
+--------+----------+

第二种：

mysql> select ename,job from emp where job in('MANAGER','SALESMAN');
+--------+----------+
| ename  | job      |
+--------+----------+
| ALLEN  | SALESMAN |
| WARD   | SALESMAN |
| JONES  | MANAGER  |
| MARTIN | SALESMAN |
| BLAKE  | MANAGER  |
| CLARK  | MANAGER  |
| TURNER | SALESMAN |
+--------+----------+

第三种就是使用union

select ename,job from emp where job = 'MANAGER'
union
select ename,job from emp where job = 'SALESMAN';
+--------+----------+
| ename  | job      |
+--------+----------+
| JONES  | MANAGER  |
| BLAKE  | MANAGER  |
| CLARK  | MANAGER  |
| ALLEN  | SALESMAN |
| WARD   | SALESMAN |
| MARTIN | SALESMAN |
| TURNER | SALESMAN |
+--------+----------+

union的作用：可以使两张毫不相干的表的查询结果拼接在一起输出，前提是两个查询的列数要相同。