3

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 则不显示任何值或 NULL

total_student   school_name

你能告诉我怎么做吗？

谢谢 ：）

4

3 回答 3

7

首先，您在GROUP BY查询底部缺少一个子句来分组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，如果您只想不显示 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，您可以更改LEFT JOIN为INNER JOIN在这种情况下完成相同的事情。

最后，如果您想将 0 替换为 null 但仍有行，则可以更新 select 语句以获取总计：

SELECT IF(COUNT(student_name) = 0, NULL, COUNT(student_name)) AS total_student, school_name
于 2013-02-03T17:21:47.103 回答
1

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
于 2013-02-03T17:26:25.100 回答
0

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

于 2013-02-03T17:25:48.627 回答