[英]MySQL - Do I need a nested query?
I need to write a query that displays, for each activity, a count of the number of males and number of females. 我需要编写一个查询,为每个活动显示男性和女性人数的计数。
Preferably in a 3 column format, activity_name, male (computed column), female (computed column). 最好采用3列格式,activity_name,男性(计算列),女性(计算列)。
I think I may need to use nested queries, but I cannot figure out how. 我想我可能需要使用嵌套查询,但是我不知道怎么做。 Or could this be done by just using GROUP BY? 还是可以仅使用GROUP BY来完成?
My tables are: 我的表是:
Child child_id child_fname child_sname child_gender etc etc 孩子 child_id child_fname child_sname child_gender等
Activity activity_id activity_name etc etc 活动 activity_id activity_name等
Child_Activity_Item child_id activity_id Child_Activity_Item child_id activity_id
Thank you 谢谢
this is what you are looking for: 这是您要寻找的:
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
you cannot directly get a three-column result because there is only only column that holds the gender. 您无法直接获得三列结果,因为只有一列包含性别。
Try this query - 试试这个查询-
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
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