Question

在我的数据库中，有2个不同的字段

我。）员工 ii。）部门

在我的employee表中，

NAME      Department
---------------------
John          IT
Siti       Research
Jason      Research

在我的Department，

Name
------------
IT
Research
Computer

使用声明

SELECT DEPARTMENT.DNAME
FROM DEPARTMENT,
     EMPLOYEE
WHERE DEPARTMENT.DNAME = EMPLOYEE.DNAME
  AND
    (SELECT COUNT(*)
     FROM EMPLOYEE.DNAME)=0;

当department中没有员工显示

时

Name
--------------
Computer

继续尝试，但有一些错误

Answer 1

两种选择：

使用IN：

SELECT name FROM Department
WHERE name NOT IN (SELECT DISTINCT Department 
                   FROM Employee)

使用Left Join：
```
SELECT D.NAME 
FROM DEPARTMENT D LEFT JOIN EMPLOYEE ON D.NAME = EMPLOYEE.Department
WHERE EMPLOYEE.Department IS NULL
```
Fiddle中的示例。

如果您的表中有数千条记录，此方法将显示出比其他方法更高的性能。

Answer 2

试试NOT IN。子查询需要DISTINCT以避免将来出现性能问题：

SELECT name FROM Department
WHERE name NOT IN ( SELECT DISTINCT Department FROM Employee);

或NOT EXIST，在大多数情况下更快：

SELECT name FROM Department
WHERE NOT EXIST  ( SELECT 1 FROM Employee 
                   WHERE Employee.Department = Department.name);

Answer 3

您没有说明您的错误，但您可以选择未显示在员工表中的所有部门：

SELECT DEPARTMENT.DNAME 
FROM DEPARTMENT 
WHERE DEPARTMENT.DNAME NOT IN (SELECT DEPARTMENT FROM EMPLOYEE);

Answer 4

SELECT D.Name
FROM Employee E
RIGHT JOIN Department D
ON E.Department=D.Name
WHERE E.Department IS NULL

SQL Fiddle DEMO

SQL计数与SELECT子句中的另一个表

4 个答案: