Question

部门表

DEPARTMENT_ID
| 1       |
| 2       |

员工表

EMPLOYEE_ID | DEPARTMENT_ID
ANDY        | 1

输出

DEPARTMENT_ID|
2

这是我的代码：

SELECT DEPARTMENT_ID
FROM DEPARTMENTS
where department_id!= all ( SELECT department_id
               FROM employees
           );

该代码未显示2，并且输出为空白。

赞：

DEPARTMENT_ID|

Answer 1

如果要查找每个部门中每个雇员的 number ，则应使用左联接聚合：

SELECT d.DEPARTMENT_ID, COUNT(e.EMPLOYEE_ID) AS cnt
FROM DEPARTMENT d
LEFT JOIN EMPLOYEES e
    ON d.DEPARTMENT_ID = e.DEPARTMENT_ID
GROUP BY d.DEPARTMENT_ID;

上面的count表达式对employee表中的EMPLOYEE_ID列进行计数。重要的是要注意，如果一个部门没有雇员，那么COUNT将返回零，因为不计算NULL。

Answer 2

SELECT d.department_id 
FROM departments d
LEFT JOIN employees e ON d.department_id = e.department_id
WHERE e.department_id IS NULL

SQLFiddle Demo

在每个部门找不到员工

2 个答案: