Question

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

我想要实现的是，如果total_student = 0则显示没有值或NULL

total_student   school_name

你能告诉我怎么做吗？

谢谢：）

Answer 1

首先，您在查询底部缺少GROUP BY子句，要按school_name进行分组：

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，如果你想简单地不显示total_student = 0的行，那么你可以使用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LEFT JOIN更改为INNER JOIN，以便在这种情况下完成相同的操作。

最后，如果你想要用0替换0但仍然有行，你可以更新select语句，使总数达到：

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

Answer 2

添加HAVING子句以过滤掉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

请尝试使用CASE语法：http://dev.mysql.com/doc/refman/5.0/en/control-flow-functions.html

如果Count（*）为零，则返回NULL

3 个答案: