将查询转换为左联接（？）以包括不匹配的用户ID

Question

我正在尝试使用MySQL查询在Bowl Bowl Mania游戏中建立用户排行榜（每个用户都试图预测每场大学橄榄球比赛的获胜者，并为每个选择分配权重）。

我有3张桌子：

--games
id   team1   team2   year

--picks
userid  gameid  points

--actResults
gameid   winner

我想构建一个SELECT查询，根据每个用户对每个成功预测的游戏的“积分”下注顺序进行排序（即，对gameid = 1的成功下注5可获得5分，而失败的预测则可获得0）。

下面的查询几乎完成了我想要的，但是唯一的问题是它不包括零分的用户（即picks.winners和actResults.winners之间没有任何匹配的用户）。

SELECT DISTINCT picks.userid AS Player, SUM( picks.points ) AS Points
FROM picks, games, actResults
WHERE picks.gameid = games.id
AND picks.winner = actResults.winner
AND games.year =  '2014'
AND picks.userid = picks.userid
GROUP BY picks.userid
ORDER BY Points DESC

期望的结果看起来像这样

Player  Points
User1   10
User4   9
User3   4
User2   0
User5   0

Answer 1

困难的部分不是转换为LEFT JOIN （基本上只涉及将逗号LEFT JOIN更改为常规ANSI JOIN ，然后插入关键字LEFT ），而是修复SUM表达式以仅将行添加到actResults.winner = picks.winner 。 这是一种实现方法：

SELECT picks.userid AS Player,
       SUM(CASE WHEN actResults.winner IS NULL
                THEN 0
                ELSE picks.points
            END) AS Points
  FROM picks
  JOIN games
    ON games.id = picks.gameid
  LEFT
  JOIN actResults
    ON actResults.winner = picks.winner
 WHERE games.year = '2014'
 GROUP BY picks.userid
 ORDER BY Points DESC
;

（另一种方法是完全跳过LEFT JOIN ，并在子查询中使用EXISTS表达式。）

---

编辑添加：要添加“ Available Points”字段，如评论中所要求，您可以编写：

SELECT picks.userid AS Player,
       SUM(CASE WHEN actResults.winner = picks.winner
                THEN picks.points
                ELSE 0
            END) AS Points,
       SUM(CASE WHEN actResults.gameid IS NULL
                THEN picks.points
                ELSE 0
            END) AS AvailablePoints
  FROM picks
  JOIN games
    ON games.id = picks.gameid
  LEFT
  JOIN actResults
    ON actResults.gameid = games.id
 WHERE games.year = '2014'
 GROUP BY picks.userid
 ORDER BY Points DESC
;