排序在SQL

Question

总值比方说，我有以下的学生数据：

classroom  gender student_id 
------------------------------------- 
First Grade M  123 
First Grade F  124 
First Grade F  125 
Second Grade M  126 
Third Grade M  127 
... 

我想产生以下结果：顶部由学生与细节每个总数下令3个最大的教室：

classroom   boys_count girls_count total_count 
-------------------------------------------------- 
Third Grade  30   30   60 
First Grade  20   5   25 
Fourth Grade  10   10   20 

我怎么能在SQL中做到这一点？如有必要，我可以使用特定的postrges功能。

---

我试过至今：在一些脚本语言

SELECT count(*) as total_count 
    , gender 
    , classroom 
ORDER BY 1 
GROUP BY classroom, gender 
LIMIT 3 

然后我重新组织的结果。但是这太慢了。我想用一个查询获得正确的结果

Answer 1

select classroom as name, 
     sum(case when gender = 'M' then 1 else 0 end) as boys_count, 
     sum(case when gender = 'F' then 1 else 0 end) as girls_count, 
     count(*) as total_count 
from your_table 
group by classroom 
order by count(*) desc 
limit 3