union合并查询结果集

案例：找出工作岗位是SALESMAN和MANAGER的员工？
select ename,job from emp where job = 'SALESMAN' or job = 'MANAGER';
select ename,job from emp where job in('SALESMAN','MANAGER');
                +--------+----------+
                | ename  | job      |
                +--------+----------+
                | ALLEN  | SALESMAN |
                | WARD   | SALESMAN |
                | JONES  | MANAGER  |
                | MARTIN | SALESMAN |
                | BLAKE  | MANAGER  |
                | CLARK  | MANAGER  |
                | TURNER | SALESMAN |
                +--------+----------+
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  |
                +--------+----------+
//union的效率要高一些，
对于表连接来说，每连接一次新表，则匹配的次数满足笛卡尔积，成倍的翻
但是union可以减少匹配的次数，在减少匹配次数的情况下，还可以完成两个结果集的拼接
a 连接 b 连接 c 
a 10条记录
b 10条记录
c 10条记录
匹配次数：10 * 10 * 10 = 1000
a 连接 b 一个结果：10 * 10 = 100
a 连接 c 一个结果：10 * 10 = 100
使用union的话是： 100 + 100 = 200  (union将乘法运算变成了加法运算 )            
 union使用注意事项
 //错误：union在进行结果集合并时，要求两个结果集的列数相同
select ename,job from emp where job = 'SALESMAN'
union
select ename from emp where job = 'MANAGER';
//MySQL可以，oracle语法严格，不可以，会报错 
//要求：结果集合并时列和列的数据类型也要一致
select ename,job from emp where job = 'SALESMAN'
union
select ename,sal from emp where job = 'MANAGER';
                +--------+----------+
                | ename  | job      |
                +--------+----------+
                | ALLEN  | SALESMAN |
                | WARD   | SALESMAN |
                | MARTIN | SALESMAN |
                | TURNER | SALESMAN |
                | JONES  | 2975     |
                | BLAKE  | 2850     |
                | CLARK  | 2450     |
                +--------+----------+