Question

我们有一个工具，允许用户创建自己的组。在这些组中，用户可以撰写帖子。我想要确定的是该组的大小与该组中的帖子总数之间的关系。

我可以使用SQL语句来获取组名列表和该组中的用户数（查询1）以及组名列表和帖子数（查询2）但我希望两者都是在同一个查询中。

查询1

select count(pg.personID) as GroupSize, g.GroupName
from Group g inner join PersonGroup pg g.GroupID = pg.GroupID
where LastViewed between @startDate and @enddate and
    g.Type = 0
group by g.GroupID, g.GroupName
order by GroupSize

查询2

select count(gp.PostID) as TotalPosts, g.GroupName
from Group g inner join GroupPost gp on g.GroupID = gp.GroupID
    inner join Post p on gp.PostID = p.PostID
where g.Type = 0 and
    gp.Created between @startDate and @enddate
group by g.GroupID, g.GroupName
order by TotalPosts

**注意：一个人可以将相同的“帖子”发布到多个组

我相信，根据这些数据，我可以建立一个直方图（10-20个用户的组数，21-30个用户等），并在这些不同的垃圾箱中合并组的平均帖子数。

Answer 1

一个简单的解决方案是将这些查询用作Sub查询，并将它们组合起来：

SELECT 
    grps.GroupName,
    grps.GroupSize,
    psts.TotalPosts
FROM (
    select count(pg.personID) as GroupSize, g.GroupName, g.GroupID
    from Group g inner join PersonGroup pg g.GroupID = pg.GroupID
    where LastViewed between @startDate and @enddate and
        g.Type = 0
    group by g.GroupID, g.GroupName
    order by GroupSize) grps
JOIN (
    select count(gp.PostID) as TotalPosts, g.GroupName, g.groupID
    from Group g inner join GroupPost gp on g.GroupID = gp.GroupID
        inner join Post p on gp.PostID = p.PostID
    where g.Type = 0 and
        gp.Created between @startDate and @enddate
    group by g.GroupID, g.GroupName
    order by TotalPosts) psts
ON psts.GroupID = grps.GroupID

Answer 2

Paul的解决方案假设两组（按帖子和用户）是相同的。这可能不是真的，因此需要完全外连接或联合。

我的偏好如下：

with groups as 
(
   select *
   from Group g
   where g.Type = 0 
     and g.LastViewed between @startDate and @enddate
)
select GroupId, GroupName, SUM(GroupSize) as GroupSize, SUM(TotalPosts) as TotalPosts)
from 
(
  (select groups.GroupId, groups.GroupName, 1 as GroupSize, 0 as TotalPosts
   from groups 
   join PersonGroup pg 
     on pg.GroupId = groups.groupId
   ) 
   union all
   (select groups.GroupId, groups.GroupName, 0 as GroupSize, 1 as TotalPosts
    from groups 
    join GroupPost gp
      on groups.GroupId = gp.GroupId 
    join Post p
      on gp.PostId = p.PostId
    )
)
group by GroupId, GroupName

“with”子句定义您正在使用的组的集合。这将定义放在一个地方，很明显两个子查询具有相同的过滤。这两个子查询只有标志，指示两个变量中的每一个，然后在更高级别汇总。有时在子查询中进行聚合也更有效，特别是在有索引时。

对同一查询中的不同表进行多次计数

2 个答案: