sql - PostgreSQL - 左连接错误计数输出

Question

我正在尝试设置一组简单的表格来显示锦标赛的结果 - 我有以下结构:

CREATE TABLE players(
    id SERIAL PRIMARY KEY,
    name TEXT);

CREATE TABLE matches(
    id SERIAL PRIMARY KEY,
    player_one_id INTEGER REFERENCES players,
    player_two_id INTEGER REFERENCES players,
    winner_id INTEGER REFERENCES players);

并且我输入了一些测试数据，如下:

INSERT INTO players (name) VALUES ('Mike Jones');
INSERT INTO players (name) VALUES ('Albert Awesome');
INSERT INTO players (name) VALUES ('Sad Sally');
INSERT INTO players (name) VALUES ('Lonely Lenny');

INSERT INTO matches (player_one_id, player_two_id, winner_id) VALUES (1,2,1);
INSERT INTO matches (player_one_id, player_two_id, winner_id) VALUES (3,4,4);

我正在尝试执行一个查询，为每个玩家提供以下结果:

id, name, matched_won, matches_played.

到目前为止，我有以下查询:

SELECT players.id, players.name, count(matches.winner_id) as matches_won
                               , count(matches.id) as matches_played
    FROM players left join matches
    ON players.id = matches.winner_id
GROUP BY players.id
ORDER BY matches_won DESC

而且，不幸的是，我得到如下不正确的输出(每个玩家应该有 1 个 matches_played):

 id |      name      | matches_won | matches_played 
----+----------------+-------------+----------------
  4 | Lonely Lenny   |           1 |              1
  1 | Mike Jones     |           1 |              1
  2 | Albert Awesome |           0 |              0
  3 | Sad Sally      |           0 |              0
(4 rows)

现在，我知道这个错误输出的原因是因为加入了 players.id = matches.winner_id，但是，我的问题是:

是否有可能只用 一个 左连接查询得到这些结果？如果是这样，如何？如果可能，我想避免进行多次查询。

Answer 1

是的。首先，您需要了解 count() 只是计算具有非 NULL 值的行数，因此您的两个计数应该相同。

要获得获胜者，请使用条件聚合:

SELECT p.id, p.name,
       sum(case when m.winner_id = p.id then 1 else 0 end) as matches_won,
       count(m.id) as matches_played
FROM players p left join
     matches m
     ON p.id in (m.player_one_id, m.player_two_id)
GROUP BY p.id
ORDER BY matches_won DESC;

您还需要修复 join 条件。您不能只加入获胜者并期望获得所有比赛的计数。