SELECT COUNT返回0

Question

有表格Departments

CREATE TABLE Departments
(
    id int NOT NULL IDENTITY(1, 1) PRIMARY KEY,
    name  nvarchar(100) NOT NULL,
    parentDepId int
);

我需要计算parentDepId = NULL的行数，但我的查询每次都返回零。

SELECT COUNT(id) as DepartmentsCount
from Departments
WHERE parentDepId = NULL;

它出了什么问题？

Answer 1

您需要使用IS NULL。

SELECT COUNT(id) AS DepartmentsCount FROM Departments 
 WHERE parentDepId IS NULL

这是与NULL比较的唯一方法（其他运算符不起作用）。

Answer 2

SELECT COUNT(Isnull(id,1)) AS DepartmentsCount
 FROM Departments 
 WHERE parentDepId IS NULL

Answer 3

我使用语法where Column is null。与等号的比较似乎是错误的