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Command:

SELECT class_id, SUM(sex = '男') AS male, SUM(sex = '女') AS female FROM student
GROUP BY class_id

Result:

Original data:

select * from student;

The data is wrong, isn’t it? If the classid is 2, there should be only 2 boys o(╯□╰)o

My answer is based on the original poster’s knowledge, and of course mine too:
First find out the situation of boys, such as t1:

|class_id|male|
|:---:|:---:|
|1|2|
|2|1|
Then find out the girl’s situation, such as t2:

Finally, chain the two watches together: select t1.class_id,male,female from t1,t2 where t1.class_id = t2.class_id

I haven’t tried it, but it probably means the right thing