MySQL：员工没有项目和没有员工的项目

Question

我想取消所有没有项目的员工和没有员工的项目。我不知道我在哪里做错了。

表结构：

员工：EMPLOYEE_ID（PK），employee_name //所有员工

项目：project_code（PK），PROJECT_TITLE //所有项目

project_team：project_code（FK） employee_id（FK）//项目和员工的桥接表正在使用中。

这是我的查询：

SELECT * 
FROM employee e 
    LEFT JOIN project_team pt USING (employee_id) WHERE pt.`project_code` IS NULL 
    INNER JOIN project p USING (project_code) WHERE (e.`employee_id` IS NULL) 

Answer 1

你可能想是这样的：

SELECT employee_id AS id_or_code, employee_name AS name_or_title FROM employee e LEFT JOIN project_team pt ON e.employee_id = pt.employee_id WHERE pt.project_code is NULL 

UNION ALL 

SELECT project_code AS id_or_code, project_title AS name_or_title FROM project p LEFT JOIN project_team pt ON p.project_code = pt.project_code WHERE pt.employee_id is NULL; 

这种方式，你有两列第一列是员工的id或查询结果代码或项目，第二列作为员工的名称或项目的标题。

Answer 2

您需要FULL JOIN，但MySQL没有它。

在这里你可以找到联合解决方案https://stackoverflow.com/a/4796911/3970250

Answer 3

您正试图合并2个不同的选择记录到一个结果集，一个方法是使用UNION ALL，然后标记每个记录类型的东西作为

select 
e.employee_id as id , 
e.employee_name as name, 
'Employee' as type 
from employee e 
left join project_team pt on pt.employee_id = e.employee_id 
where pt.employee_id is null 
union all 
select 
p.project_code as id , 
p.project_title as name, 
'Project' as type 
from project p 
left join project_team pt on pt.project_code = p.project_code 
where pt.project_code is null ; 

下面是测试情况

mysql> select * from employee ; 
+-------------+---------------+ 
| employee_id | employee_name | 
+-------------+---------------+ 
|   1 | A    | 
|   2 | B    | 
|   3 | C    | 
|   4 | D    | 
|   5 | E    | 
+-------------+---------------+ 
5 rows in set (0.00 sec) 

mysql> select * from project ; 
+--------------+---------------+ 
| project_code | project_title | 
+--------------+---------------+ 
|   1 | P1   | 
|   2 | P2   | 
|   3 | P3   | 
|   4 | P4   | 
+--------------+---------------+ 
4 rows in set (0.00 sec) 

mysql> select * from project_team ; 
+--------------+--------------+ 
| project_code | employee_id | 
+--------------+--------------+ 
|   1 |   1 | 
|   1 |   2 | 
|   2 |   3 | 
|   2 |   4 | 
|   3 |   2 | 
|   3 |   1 | 
|   3 |   3 | 
+--------------+--------------+ 

运行上面的查询将会给你

+------+------+----------+ 
| id | name | type  | 
+------+------+----------+ 
| 5 | E | Employee | 
| 4 | P4 | Project | 
+------+------+----------+