数据库中查询一个男生比女生多的班级数?怎么用查询? 求代码。

数据库中查询一个男生比女生多的班级数?怎么用查询? 求代码。

你没有说明数据库结构，假如数据库结构如下：

TB_ClASS（班级数据表，主键：class_id）
-------------------------
| class_id | class_name |
-------------------------

TB_STUDENTS（学生信息表，主键：student_id，外键：class_id）
-------------------------------
| student_id | sex | class_id |
-------------------------------

那么建议先按照班级建立视图，然后从视图中查询。
建立视图：
SELECt TB_STUDENT.class_id, TB1.male AS male, TB2.female
FROM TB_STUDENT LEFT OUTER JOIN
(SELECt class_id, COUNT(student_id) AS male
FROM TB_STUDENT AS TB_STUDENT_1
WHERe (student_sex = '男')
GROUP BY class_id) AS TB1 ON TB_STUDENT.class_id = TB1.class_id LEFT OUTER JOIN
(SELECt class_id, COUNT(student_id) AS female
FROM TB_STUDENT AS TB_STUDENT_2
WHERe (student_sex = '女')
GROUP BY class_id) AS TB2 ON TB_STUDENT.class_id = TB2.class_id
GROUP BY TB_STUDENT.class_id, TB1.male, TB2.female

得到如下视图形式：
class_sex_view
----------------------------
| class_id | male | female |
----------------------------

再用以下简单查询即可：
select count(class_id)
from class_sex_view
where male>female

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