Question

我想计算每个conversations关闭了多少user，以及每个messages写了多少user。

user有许多conversations

conversations有许多messages

message可以属于user

这是我有点远的查询

select a.id, u.display_name, count(c.id) as closed, count(m.id) as replied
from apps a
left join app_users au on au.app_id = a.id
left join users u on u.id = au.user_id
left join conversations c on c.app_id = a.id and c.closed_by_id = u.id
left join messages m on m.conversation_id = c.id and m.user_id = u.id
group by a.id, u.id
order by closed desc

当我没有加入消息并只计算已关闭的对话时，它工作正常。加入邮件时，closed和replied列的编号完全相同（并且两者都不正确）

有什么想法吗？

Answer 1

您可以在加入之前在子查询中进行计数：

select a.id, u.display_name, c.closed, m.replied
from apps a
left join app_users au on au.app_id = a.id
left join users u on u.id = au.user_id
left join lateral (
    select id, count(*) as closed
    from conversations
    where closed_by_id = u.id) c on c.app_id = a.id
left join lateral (
    select count(*) as replied
    from messages
    where user_id = u.id) m on m.conversation_id = c.id
order by c.closed desc;

Answer 2

快速而肮脏的解决方案是使用count(distinct)：

select a.id, u.display_name,
       count(distinct c.id) as closed, count(distinct m.id) as replied

这在许多情况下都有效。但如果有很多＆＃34;关闭＆＃34;并且＆＃34;回复＆＃34;，然后中间计算可能非常大，影响性能。如果这是一个问题，那么在联接之前预先聚合结果就是解决方案。

计算关联数量

2 个答案: