为什么SQL查询SELECT不返回IS NULL结果

Question

我需要显示在同一部门没有指定主管的员工列表

Table: employee
Fields:
id INT(pk),
department_id INT,
chief_id INT,
name Varchar(100),
salary INT

查询：

SELECT One.name AS Employee,One.department_id, Two.name AS Chief, Two.department_id

FROM employee One, employee Two

WHERE (One.chief_id = Two.id AND One.department_id != Two.department_id) OR One.department_id IS NULL;

Answer 1

SELECT One.name AS Employee,One.department_id, Two.name AS Chief,  Two.department_id
FROM employee One
LEFT JOIN employee two ON One.chief_id = Two.id
WHERE NOT EXISTS (
                 SELECT 0 FROM employee two 
                     where One.department_id = Two.department_id 
                            AND One.chief_id = Two.id
                )

Answer 2

尝试跟随；）

SELECT One.name AS Employee,One.department_id, Two.name AS Chief, Two.department_id
FROM employee One, employee Two
WHERE One.chief_id = Two.id AND (One.department_id IS NULL OR One.department_id <> Two.department_id);

Answer 3

下面的查询可能会提供所需的结果：

SQL服务器 ：-

SELECT One.name AS Employee,One.department_id, Two.name AS Chief, Two.department_id
FROM employee One, employee Two
WHERE (One.chief_id = Two.id AND One.department_id != Two.department_id) OR ISNULL(One.department_id,0)=0;

MYSQL ：-

SELECT One.name AS Employee,One.department_id, Two.name AS Chief, Two.department_id
FROM employee One, employee Two
WHERE (One.chief_id = Two.id AND One.department_id != Two.department_id) OR IFNULL(One.department_id,0)=0;

Answer 4

尝试查询

SELECT 
  Two.name AS Employee,
  Two.department_id AS Employee_Dept,
  One.name AS Chief_Name,
  One.department_id AS Chif_Dept 
FROM
  employee ONE 
  INNER JOIN employee Two 
    ON One.id = Two.chief_id 
    AND One.department_id != Two.department_id ;