使用 group by、2 having 和 where 子句连接 4 个表

Question

我的数据库包含 4 个表：

users(id, "name", 姓氏, 生日)
友谊（userid1，userid2，"timestamp"）
posts(id, userid, "text", "timestamp")
喜欢（postid，userid，"timestamp"）

我需要获得一个结果集，其中包含在 2018 年 1 月内拥有超过 3 个友谊的唯一用户名，并且他们的 "likes" 平均每个 "post" 在 [10; 35).

我为第一步写了这个语句：

select  distinct u."name"
from users u
join friendships f on u.id = f.userid1
where f."timestamp" between '2018-01-01'::timestamp and '2018-01-31'::timestamp
group by u.id
having count(f.userid1) > 3;

工作正常，returns 3 行。但是当我以这种方式添加第二部分时：

select  distinct u."name"
from users u
join friendships f on u.id = f.userid1
join posts p on p.userid = u.id
join likes l on p.id = l.postid
where f."timestamp" between '2018-01-01'::timestamp and '2018-01-31'::timestamp
group by u.id
having count(f.userid1) > 3 
    and ((count(l.postid) / count(distinct l.postid)) >= 10 
        and (count(l.postid) / count(distinct l.postid)) < 35);

我快疯了 94 行。我不知道为什么。将感谢可能的帮助。

Answer 1

试试下面的方法！使用 "count(f.userid1) > 3" 的问题是，如果用户有，例如2 个朋友和 6 个帖子和 3 个赞他们将得到 2 x 6 = 12 行，因此 12 条记录具有非空 f.userid1。通过计算不同的 f.userid2，您可以计算不同的朋友。用于过滤的其他计数也会出现类似问题。

select  u."name"
from users u
join friendships f on u.id = f.userid1
join posts p on p.userid = u.id
left join likes l on p.id = l.postid
where f."timestamp" > '2018-01-01'::timestamp and f."timestamp" < '2018-02-01'::timestamp
group by u.id, u."name"
having
 --at least three distinct friends
 count( distinct f.userid2) > 3 
  --distinct likes / distinct posts
  --we use l.* to count distinct likes since there's no primary key
  and ((count(distinct l.*) / count(distinct p.id)) >= 10 
        and ((count(distinct l.*) / count(distinct p.id)) < 35);

Answer 2

您不需要 u.name 中的 distinct，因为聚合会删除重复项。

select
   u."name"
from 
   users u
   inner join friendships f on u.id = f.userid1
   inner join posts p on u.id = p.userid
   inner join likes l on p.id = l.postid
where 
   f."timestamp" >= '2018-01-01'::timestamp 
   and f."timestamp" < '2018-02-01'::timestamp
group by 
    u."name"
having 
    count(distinct f.userid1) > 3 
    and ((count(l.postid) / count(distinct l.postid)) >= 10 
            and (count(l.postid) / count(distinct l.postid)) < 35);

如评论所述。当你使用 between 代替 date 做范围时不是个好主意。

f."timestamp" >= '2018-01-01'::timestamp 
and f."timestamp" < '2018-02-01'::timestamp

会给你一个完整的一月。

使用 group by、2 having 和 where 子句连接 4 个表

Join 4 tables with group by, 2 having and where clause

postgresql

join

group-by

having

having-clause