協會計數

Question

我想統計每user已關閉多少個conversations，以及每個user已編寫多少個messages。

一個user有很多conversations

一個conversations有許多messages

一個message可以屬於user

這是查詢我有些遠

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 

當我沒有加入消息時，它工作正常只是計算封閉的談話。加入郵件時，closed和replied列是完全相同的數字（對於兩者也是不正確的）

任何想法？

Answer 1

快速和骯髒的解決方案是使用count(distinct)：

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

這將在很多情況下工作。但是，如果有很多「封閉」和「回答」，則中間計算可能會相當大，從而影響性能。如果這是一個問題，那麼在聯接之前預先彙總結果是解決方案。

Answer 2

你可以做加盟之前，子查詢計數：

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;