Mysql SQL嵌套计数查询

我有这些桌子：

Discussion
----------
DiscussionID (Primary Key)
Name
....

UserDiscussion
--------------
UserID (Primary Key)
DiscussionID (Primary Key)
Participated (Boolean)
Bookmarked (Boolean)

我需要询问两件事：

某个用户参与的所有讨论

对于每个参与的讨论，我需要将讨论添加到书签的用户总数

以下是我当前的查询：

SELECT d.DiscussionID, d.Name
FROM Discussion d LEFT JOIN
     UserDiscussion ud ON ud.DiscussionID = d.DiscussionID
WHERE ud.UserID = 1 AND ud.Participated = true;

样本数据：

Discussion:

DiscussionID | Name |
---------------------
1              First

UserDiscussion:

UserID | DiscussionID | Participated | Bookmarked |
===================================================
    1          1              1              1
    4          1              1              0

输出应如下所示：

讨论：[ { 讨论ID:1， 名称：“第一”， 书签计数：1 }

---

]

您可以将in子句与子层、、和计数一起使用

SELECT a.DiscussionID, a.Name, b.count(*)
FROM discussion a
INNER JOIN  UserDiscussion b 
WHERE a.DiscussionID in (
    SELECT d.DiscussionID
    FROM Discussion d
    INNER JOIN  UserDiscussion ud ON ud.DiscussionID = d.DiscussionID
    WHERE ud.UserID = 1 AND ud.Participated = true
)
AND b.Bookmarked  = true;
group by a.DiscussionID, a.Name

---

听起来你有第一个查询，2d应该是这样的

SELECT DiscussionID, count(UserID)
FROM UserDiscussion        
WHERE DiscussionID in (SELECT DiscussionID from UserDiscussion WHERE ud.UserID = 1 AND ud.Participated = true)
      AND Bookmarked = true
GROUP BY DiscussionID;

最后你需要加入他们

与张贴的更正：

SELECT d.DiscussionID, d.Name, BookmarkCount
FROM Discussion d
JOIN UserDiscussion ud ON ud.DiscussionID = d.DiscussionID
JOIN (
SELECT DiscussionID as DID, count(UserID) as BookmarkCount
FROM UserDiscussion
WHERE DiscussionID in (SELECT DiscussionID from UserDiscussion WHERE
ud.UserID = 1 AND ud.Participated = true)
      AND Bookmarked = true
GROUP BY DiscussionID
) ON d.DiscussionID=DID;

---

一种方法是编写一个包含书签次数的视图

创建或替换已添加书签的视图(
讨论会，
泰晤士报（书签）为
选择讨论id，计数（*）
来自用户讨论
其中Bookmarked=true
分组讨论\u id

---

然后在主查询中，左键连接到视图。

您可以使用临时表获得与每个讨论ID相关的书签计数

select DiscussionID, count(Bookmarked) as 'Bookmarked'
into #Bookmarks
from UserDiscussion
group by DiscussionID

然后，将其连接到DiscussionID上的最后一个表，您可以将计数用作列

SELECT d.DiscussionID, d.Name, b.Bookmarked
FROM Discussion d LEFT JOIN
     UserDiscussion ud ON ud.DiscussionID = d.DiscussionID
left join #Bookmarked b on ud.DiscussionID = b.DiscussionID
WHERE ud.UserID = 1 AND ud.Participated = true;

---

输出应该是什么样子？这是一个

mysql

问题还是

postgresql

问题？或者您正在寻找这两个系统的答案？我更新了输出应该是

DiscussionID

之后的

where

正确，还是应该是

d.DiscussionID

？

SELECT d.DiscussionID, d.Name, b.Bookmarked
FROM Discussion d LEFT JOIN
     UserDiscussion ud ON ud.DiscussionID = d.DiscussionID
left join #Bookmarked b on ud.DiscussionID = b.DiscussionID
WHERE ud.UserID = 1 AND ud.Participated = true;