汇总带有其他（不同）过滤器的列

Question

这段代码可以正常工作，但是我又长又令人毛骨悚然。

select p.name, p.played, w.won, l.lost from

(select users.name, count(games.name) as played
from users
inner join games on games.player_1_id = users.id
where games.winner_id > 0
group by users.name
union
select users.name, count(games.name) as played
from users
inner join games on games.player_2_id = users.id
where games.winner_id > 0
group by users.name) as p

inner join

(select users.name, count(games.name) as won
from users
inner join games on games.player_1_id = users.id
where games.winner_id = users.id
group by users.name
union
select users.name, count(games.name) as won
from users
inner join games on games.player_2_id = users.id
where games.winner_id = users.id
group by users.name) as w on p.name = w.name

inner join

(select users.name, count(games.name) as lost
from users
inner join games on games.player_1_id = users.id
where games.winner_id != users.id
group by users.name
union
select users.name, count(games.name) as lost
from users
inner join games on games.player_2_id = users.id
where games.winner_id != users.id
group by users.name) as l on l.name = p.name

如您所见，它由3个重复部分组成，用于检索：

• 玩家名称和他们玩过的游戏数量
• 玩家名称和他们赢得的游戏数量
• 玩家姓名和输掉的游戏数量

并且每个还包括2个部分：

• 玩家名称和以玩家_1身份参加的游戏数量
• 玩家名称以及他们作为玩家_2参加的游戏数量

如何简化呢？

结果看起来像这样：

           name            | played | won | lost 
---------------------------+--------+-----+------
 player_a                  |      5 |   2 |    3
 player_b                  |      3 |   2 |    1
 player_c                  |      2 |   1 |    1

Answer 1

Postgres 9.4* 或更高版本中的 聚合FILTER子句越来越短： *

SELECT u.name
     , count(*) FILTER (WHERE g.winner_id  > 0)    AS played
     , count(*) FILTER (WHERE g.winner_id  = u.id) AS won
     , count(*) FILTER (WHERE g.winner_id <> u.id) AS lost
FROM   games g
JOIN   users u ON u.id IN (g.player_1_id, g.player_2_id)
GROUP  BY u.name;

在Postgres 9.3 （或 任何 版本）中，它仍然比嵌套的子选择或CASE表达式短和快：

SELECT u.name
     , count(g.winner_id  > 0 OR NULL)    AS played
     , count(g.winner_id  = u.id OR NULL) AS won
     , count(g.winner_id <> u.id OR NULL) AS lost
FROM   games g
JOIN   users u ON u.id IN (g.player_1_id, g.player_2_id)
GROUP  BY u.name;

细节：

• 为了获得绝对的性能，SUM是更快还是COUNT？