MySQL - 我需要嵌套查询吗？

Question

我需要编写一个查询，以显示每个活动的男性人数和女性人数。

最好以3列格式，activity_name，男性（计算列），女性（计算列）。

我想我可能需要使用嵌套查询，但我无法弄清楚如何。或者可以通过使用GROUP BY来完成？

我的表是：

儿童 child_id child_fname child_sname child_gender 等等等等

活动 ACTIVITY_ID ACTIVITY_NAME 等等等等

Child_Activity_Item child_id ACTIVITY_ID

谢谢

Answer 1

尝试此查询 -

SELECT 
    a.activity_id, 
    a.activity_name, 
    COUNT(IF(c.child_gender = 'M', c.child_gender, NULL)) male, 
    COUNT(IF(c.child_gender = 'F', c.child_gender, NULL)) female 
FROM 
    Activity a 
    JOIN Child_Activity_Item ca 
    ON ca.activity_id = a.activity_id 
    JOIN Child c 
    ON c.child_id = ca.child_id 
GROUP BY a.activity_id 

Answer 2

这是你在找什么：

SELECT MaleList.Activity_Name, 
     MaleList.totalMale, 
     FemaleList.totalFemale, 
FROM 
(
    SELECT a.Activity_ID, 
      a.Activity_Name, 
      COUNT(c.child_gender) totalMale 
    FROM Activity a 
       INNER JOIN Child_Activity_Item b 
        on a.activity_id = b.activity_ID 
       INNER JOIN Child c 
        on b.child_id = c.child_id 
    WHERE c.child_gender = 'male' 
    Group BY a.Activity_ID 
) as MaleList INNER JOIN 
(
    SELECT a.Activity_ID, 
      a.Activity_Name, 
      COUNT(c.child_gender) totalFemale 
    FROM Activity a 
       INNER JOIN Child_Activity_Item b 
        on a.activity_id = b.activity_ID 
       INNER JOIN Child c 
        on b.child_id = c.child_id 
    WHERE c.child_gender = 'female' 
    Group BY a.Activity_ID 
) as FemaleList 
    ON MaleList.Activity_ID = FemaleList.Activity_ID 

你不能直接获得一个三列的结果，因为只有唯一的列持有性别。