如何计算同一 table 中的两个单独的列并将它们相加到一个新列中

Question

我有两个表：playernames 和 matches.

SELECT * FROM playernames;
 id |       name       
----+------------------
 38 | Abe Lincoln
 39 | Richard Nixon
 40 | Ronald Reagan
(3 rows)

SELECT * FROM matches;
 match_id | winner | loser 
----------+--------+-------
        6 |     38 |    39
        8 |     38 |    39
        9 |     39 |    38
       10 |     38 |    39
       11 |     40 |    39
(5 rows)

我需要创建一个包含 returns 四列的查询：id、name、wins、matches。但即使使用连接和子查询，我似乎也无法在一个查询中获取所有内容。我最接近的是执行两个单独的查询。这是计算每位玩家总获胜次数的查询：

SELECT playernames.id, name, COUNT(*) AS wins 
FROM matches, playernames 
WHERE winner = playernames.id 
GROUP BY playernames.id, winner;
 id |       name       | wins 
----+------------------+------
 38 | Abe Lincoln      |    3
 39 | Richard Nixon    |    1
 40 | Ronald Reagan    |    1
(3 rows)

但是如果我想正确计算每个玩家的比赛总数，我必须发出单独的查询：

SELECT playernames.id, name, COUNT(*) 
FROM matches, playernames 
WHERE playernames.id = winner 
OR playernames.id = loser 
GROUP BY playernames.id;
 id |       name       | count 
----+------------------+-------
 40 | Ronald Reagan    |     1
 38 | Abe Lincoln      |     4
 39 | Richard Nixon    |     5
(3 rows)

我避免将这个问题与我在将这些问题组合成一个查询时所做的许多错误尝试弄得一团糟。谁能告诉我如何正确地将这两个查询合二为一？

Answer 1

我会使用一个连接来获取玩家参加的所有比赛，然后计算一个 case 表达式以仅提取他赢得的比赛：

SELECT    playernames.id, name, 
          COUNT(CASE playernames.id WHEN winner THEN 1 ELSE NULL END) AS wins, 
          COUNT(match_id) AS matches
FROM      playernames 
LEFT JOIN matches ON playernames.id IN (winner, loser)
GROUP BY  playernames.id, name;

如何计算同一 table 中的两个单独的列并将它们相加到一个新列中

How to count two separate columns in the same table and sum them into a new column

sql

postgresql

join

count

subquery