Mysql 如果计数（*）为零，则返回NULL

我有以下mysql查询：

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

我试图实现的是，如果total_student=0，则不显示任何值或NULL

total_student   school_name 

你能告诉我怎么做吗

---

谢谢：）

首先，您在查询“分组依据”

school\u name

的底部缺少一个

groupby

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

然后，如果您只想不显示total_student=0的行，那么可以使用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，您可以将

左连接

更改为

内部连接

，以在本例中完成相同的操作

最后，如果您想将0替换为null，但仍有行，则可以更新select语句，使总计为：

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

---

请尝试使用用例语法：

添加HAVING子句以筛选出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