自顶

select classId, (sum(CASE WHEN gender = '男' THEN 
                      num ELSE 0 END)) as 男,
                (sum(CASE WHEN gender = '女' THEN 
                      num ELSE 0 END)) as 女                
from (

select classId, gender, 1 as num from class_user

) b
group by classId

由于在服务端是有数据持久化的，因此只要在内存中找到这张表，然后直接查出人数即可

select classId,
sum(case when gender='男' then 1 else 0 end) as '男',
sum(case when gender='女' then 1 else 0 end) as '女'
from student
group by classId

case when 写法，时间换空间，牺牲 CPU 时间用于判断和计算
left join 写法，空间换时间，两个查询结果在内存中 sort

附 SQL：

SELECT 
  a.classid,
  IFNULL(a.male_num, 0) AS male_num,
  IFNULL(b.famale_num, 0) AS famale_num
FROM
  (
    (SELECT 
      classid,
      COUNT(*) AS male_num 
    FROM
      student 
    WHERE gender = 'M' GROUP BY classid) a 
    LEFT JOIN 
      (SELECT 
        classid,
        COUNT(*) AS famale_num 
      FROM
        student 
      WHERE gender = 'F' GROUP BY classid) b ON a.classid = b.classid
  ) 
GROUP BY a.classid ;

SELECT classid ,
SUM(IF(gender ='男',1,0)) as '男',
SUM(IF(gender ='女',1,0)) as '女'
FROM student 
GROUP BY classid;

你这个不错

为啥不用 count 呢？