將查詢轉換爲左連接（？）以包含不匹配的用戶ID

Question

我正在嘗試使用MySQL查詢來構建用戶在Bowl Mania遊戲中的排行榜（用戶每次嘗試預測每個學院的獲勝者足球比賽，併爲每個選項分配一個分數權重）。

我有3個表：

--games 
id team1 team2 year 

--picks 
userid gameid points 

--actResults 
gameid winner 

我想建立它可以根據他們對每個成功預測遊戲「點」下注夏天每個用戶一個SELECT查詢（即5一個成功的賭注在gameid = 1時獲得5分，而不成功的預測則獲得0分）。

下面的查詢幾乎完成我想要的，但唯一的問題是它不包括具有零點的用戶（即picks.winners和actResults.winners之間沒有任何匹配的用戶）。

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 

期望的結果會是這個樣子

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

Answer 1

難的是完全沒有轉換到LEFT JOIN（基本上，只是涉及到改變逗號加入到常規的ANSI JOIN，然後插入關鍵字LEFT），而是修復SUM表達式以僅添加actResults.winner = picks.winner的行。這裏是做它的一種方法：

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 
; 

（另一種方式是跳過LEFT JOIN完全，並使用EXISTS表達的子查詢。）

---

編輯補充：要添加根據評論中的要求，您可以編寫一個「可用點」字段：

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 
;