如果Count（*）為零，則返回NULL

Question

我有以下mysql查詢：

SELECT count(student_name) AS total_student,school_name FROM `student` 
LEFT JOIN school_info ON school_info.school_id=student.school_id
WHERE student.status='0'

它返回：

total_student   school_name
  0               NULL

我想要實現的是，如果total_student = 0則顯示沒有值或NULL

total_student   school_name 

你能告訴我怎么做嗎？

謝謝 ：）

Answer 1

首先，您在查詢底部缺少GROUP BY子句以按school_name ：

SELECT count(student_name) AS total_student, school_name
FROM student
    LEFT JOIN school_info ON school_info.school_id = student.school_id
WHERE student.status = '0'
GROUP BY school_name

然后，如果您只想顯示total_student = 0的行，那么您可以使用MySQL HAVING子句：

SELECT count(student_name) AS total_student, school_name
FROM student
    LEFT JOIN school_info ON school_info.school_id = student.school_id
WHERE student.status = '0'
GROUP BY school_name
HAVING count(student_name) > 0

或者，您可以將LEFT JOIN更改為INNER JOIN以在此情況下完成相同的操作。

最后，如果你想用null替換0但仍然有行，你可以更新select語句，使總數達到：

SELECT IF(COUNT(student_name) = 0, NULL, COUNT(student_name)) AS total_student, school_name

Answer 2

添加HAVING子句以過濾掉0行：

SELECT count(student_name) AS total_student,school_name FROM `student` 
LEFT JOIN school_info ON school_info.school_id=student.school_id
WHERE student.status='0'
HAVING total_student > 0

Answer 3

請嘗試使用CASE語法： http ： //dev.mysql.com/doc/refman/5.0/en/control-flow-functions.html