爲什麼是我的兩列總和PSQL不正確的？

Question

我試圖讓兩列的總和PSQL

select id as id, name as name, count(winner_id) as winner_count, 
(count(winner_id) +count(loser_id)) as total_matches from player left join 
matches on matches.winner_id = player.id group by winner_id, player.name,player.id, 
matches.loser_id;") 

總比賽數應占總或winner_id和loser_id在比賽表。

但它是不正確。

它應該返回1場，比賽爲所有玩家，而是它返回了誰贏得了玩家2場和誰失去了球員0匹配。

Answer 1

count()計數的次數，一個值不NULL。所以，這兩個count()旨意可能返回相同的值。我懷疑你想是這樣的：

select id, name, 
     sum(case when id = winner_id then 1 else 0 end) as winner_count, 
     count(m.id) as total_matches 
from player p left join 
    matches m 
    on m.winner_id = p.id 
group by p.name, p.id;