PostgreSQL - 在一列和兩列中選擇唯一值的計數

Question

首先，我想爲模棱兩可的標題道歉（我承諾一旦我真正意識到我正在嘗試解決的問題，就修改它）！

我有兩個表，球員和比賽，看起來像下面這樣：

球員：

id name 
-- ---- 
1 John 
2 James 
3 April 
4 Jane 
5 Katherine 

匹配：

id winner loser 
-- ------ ----- 
1 1  2 
2 3  4 

記錄在匹配表中贏家和失敗者表示兩個播放器，其中由所述數據庫中生成的ID柱，並且這些值之間的匹配列參考id列中的播放器表。

我想運行的吐出下面的查詢：

player.id player.name total_wins total_matches 
--------- ----------- ---------- ------------- 
1   John  1   1 
2   James  0   1 
3   April  1   1 
4   Jane  0   1 
5   Katherine 0   0 

我現在有它檢索total_wins查詢，但我不知道如何讓TOTAL_MATCHES最重要的是計數。

select p.id, p.name, count(m.winner) 
from player p left join match m on p.id = m.winner 
group by p.id, p.name; 

感謝您的幫助！

Answer 1

嘗試

select p.id, p.name, 
     sum(case when m.winner = p.id then 1 end) as total_wins, 
     count(m.id) as total_matches 
from player p 
left join match m on p.id in (m.winner, m.loser) 
group by p.id, p.name; 

Answer 2

一種方法拆分match匹配表，所以你有一個單一的行贏得和損失。剩下的只是一個left join和聚集：

select p.id, p.name, coalesce(sum(win), 0) as win, count(m.id) as total_matches 
from player p left join 
    (select match, winner as id, 1 as win, 0 as loss from match 
     union all 
     select match, loser as id, 0 as win, 1 as loss from match 
    ) m 
    on p.id = m.id 
group by p.id, p.name;