MySQL - 我需要嵌套查詢嗎？

Question

我需要編寫一個查詢，以顯示每個活動的男性人數和女性人數。

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

我想我可能需要使用嵌套查詢，但我無法弄清楚如何。或者可以通過使用GROUP BY來完成？

我的表是：

兒童 child_id child_fname child_sname child_gender 等等等等

活動 ACTIVITY_ID ACTIVITY_NAME 等等等等

Child_Activity_Item child_id ACTIVITY_ID

謝謝

Answer 1

嘗試此查詢 -

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

這是你在找什麼：

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 

你不能直接獲得一個三列的結果，因爲只有唯一的列持有性別。