[英]How to use a subquery inside another subquery with three tables in mysql?
我的數據庫中有三個表,請參閱下面的結構:
用戶
uid | fname | date_joined
---------------------------------
1 | john | 2013-08-25 01:00:00
2 | rock | 2013-08-26 01:00:00
3 | jane | 2013-08-27 01:00:00
問題
qid | uid
----------
1 | 1
2 | 1
3 | 2
4 | 3
5 | 3
6 | 1
7 | 1
8 | 2
9 | 2
追隨者
fid | qid
---------
1 | 2
2 | 1
3 | 2
4 | 1
5 | 2
6 | 3
7 | 2
用戶表包含所有與用戶相關的字段
問題表包含與外鍵uid相關的所有問題
關注者表存儲問題遵循的次數的信息
我希望我的查詢返回的是:
unique uid,
fname
question count for each user
follower count for each user
我寫了一個查詢,它的工作正常,並按我的意願返回記錄,但是追隨者計數總是0.這是我的查詢:
SELECT
u.uid, u.fname, u.date_joined ,
(SELECT COUNT(*) FROM questions WHERE questions.uid = u.uid) AS question_count,
(SELECT COUNT(*) FROM followers WHERE followers.qid IN (
SELECT GROUP_CONCAT(qid) FROM questions WHERE questions.uid = u.uid
)
) AS follow_count
FROM epc_user AS u
ORDER BY follow_count DESC, question_count DESC, date_joined DESC
我嘗試了幾種不同的組合,但它們都沒有工作,也許我正在寫一個錯誤的查詢,或者它不可能在另一個子查詢中使用子查詢,無論它是什么。 我只是想知道它是否可能,如果可能的話
您可以使用聯接來獲取結果。 不需要子查詢:
select
u.uid,
u.fname,
u.date_joined,
count( distinct q.qid ) as 'question_count',
count( distinct f.fid ) as 'follower_count'
from users u
left join questions q on q.uid = u.uid
left join followers f on f.qid = q.qid
group by u.uid, u.fname, u.date_joined
order by follower_count desc, question_count desc, date_joined desc;
GROUP_CONCAT
將返回一個字符串,但IN
運算符使用一組值而不是字符串,所以你必須使用這樣的東西
SELECT
u.uid, u.fname, u.date_joined ,
(SELECT COUNT(*) FROM questions WHERE questions.uid = u.uid) AS question_count,
(SELECT COUNT(*) FROM followers WHERE followers.qid IN (
SELECT qid FROM questions WHERE questions.uid = u.uid
)
) AS follow_count
FROM users AS u
ORDER BY follow_count DESC, question_count DESC, date_joined DESC
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