练习：查询每个班 男生女生分别有多少人

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练习：查询每个班男生女生分别有多少人

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爱竹子的大黑熊

#1 Created at ... [Delete] [Delete and Lock User]

SELECT
 c.name '班级',
 s.gender '性别',
 COUNT(s.gender) '人数'
FROM students s, classes c
WHERE c.id = s.class_id
GROUP BY s.class_id, s.gender;

![](https://raw.githubusercontent.com/tangg9646/my_github_image_bed/master/20200325171902.png)

| 一班 | M | 2 |

| ---- | ---- | ---- |

| 一班 | F | 2 |

| 二班 | F | 1 |

| 二班 | M | 2 |

| 三班 | F | 2 |

| 三班 | M | 1 |

用户06lgllpdgh

#2 Created at ... [Delete] [Delete and Lock User]

SELECT

classes.name,

students.gender sex,

COUNT(s.gender) num

FROM

students s, classes c

WHERE

s.class_id = c.id GROUP BY s.gender;

用户06lgllpdgh

#3 Created at ... [Delete] [Delete and Lock User]

# 上面错了，这个才对

SELECT

classes.name,

students.gender sex,

COUNT(s.gender) num

FROM

students s, classes c

WHERE

s.class_id = c.id

GROUP BY c.name,s.gender;

LiuChenO

#4 Created at ... [Delete] [Delete and Lock User]

select s.class_id, s.gender, c.id cid, COUNT(s.gender) num

from students s

left join classes c

on s.class_id = cid

group by s.class_id, s.gender