从多个表中选择相同字段值的所有数据

Question

这是我的数据库表的样子：

regular_employee：

+----+---------------+-----------------+-----------+----------------+
| id | employee_name | employee_job    | join_date | permanent_date |
+----+---------------+-----------------+-----------+----------------+
|  1 | ZELDA         | ACCOUNTANT      | 2020/02   | 2020/03        |
|  2 | YUGO          | QA              | 2020/02   | 2020/04        |
|  3 | XAVIER        | GENERAL MANAGER | 2020/02   | 2020/05        |
|  4 | WAVY          | FACTORY MANAGER | 2020/01   | 2020/02        |
+----+---------------+-----------------+-----------+----------------+

合同雇员：

+----+---------------+--------------+-----------+
| id | employee_name | employee_job | join_date |
+----+---------------+--------------+-----------+
|  1 | ANTONIO       | ACCOUNTANT   | 2020/01   |
|  2 | BAGGIO        | ENGINEER     | 2020/02   |
|  3 | CHARLES       | QA           | 2020/02   |
|  4 | DAVID         | QA           | 2020/02   |
+----+---------------+--------------+-----------+

我的目标： 选择 2020/02 年加入公司的所有员工职位，并按员工职位区分/分组

我尝试过的：

SELECT re.employee_job,ce.employee_job 
FROM regular_employee AS re,contract_employee AS ce 
WHERE re.join_date = '2020/02' AND ce.join_date = '2020/02' 
GROUP BY re.employee_job,ce.employee_job

结果是：

+-----------------+--------------+
| employee_job    | employee_job |
+-----------------+--------------+
| ACCOUNTANT      | ENGINEER     |
| ACCOUNTANT      | QA           |
| GENERAL MANAGER | ENGINEER     |
| GENERAL MANAGER | QA           |
| QA              | ENGINEER     |
| QA              | QA           |
+-----------------+--------------+

我所期待的：

+-----------------+
| employee_job    |
+-----------------+
| ACCOUNTANT      |
| ENGINEER        |
| GENERAL MANAGER |
| QA              |
+-----------------+

这个查询怎么做？

Answer 1

联合查询在这里可能更有意义：

SELECT employee_job FROM regular_employee WHERE join_date = '2020/02'
UNION
SELECT employee_job FROM contract_employee WHERE join_date = '2020/02';

UNION 默认情况下会删除可能出现在两个表中的一个/两个中的重复作业。