MYSQL SUM和substract

Question

我有以下SQL查詢。

SELECT u.username, SUM(p.points) AS points, 
      SUM(sp.spPoints) AS spPoints, 
      (SUM(sp.spPoints) - SUM(p.points)) AS Puntos_Restantes 
FROM  users as u 
LEFT JOIN points as p ON (u.userid = p.userid) 
LEFT JOIN sppoints AS sp ON (u.userid = sp.userid) 
WHERE  u.userid = '1' 
GROUP BY u.userid 

我的目標是要總結2場，然後減去他們，但是當我執行上面的查詢，第二SUM是錯誤的。
表是這樣的：

points: pointId, userId, points  
sppoints: spPointId, userId, spPoints  

在點我有這樣的量：25和spPoints：10 但是當我運行查詢，我得到：

points spPoints Puntos_Restantes 
25  30    5 

這是怎麼回事錯在這裏？

Answer 1

表users與其他兩個表格有一對多的關係。這會導致2個連接生成一個迷你Carstesian產品，並在points列中生成具有相同數據的多行 - 然後進行彙總。

您可以通過使用子查詢組，然後加入，以避免這個問題：

SELECT 
     u.username 
    , COALESCE(p.pPoints,0) 
     AS pPoints 
    , COALESCE(sp.spPoints,0) 
     AS spPoints 
    , COALESCE(p.pPoints,0) - COALESCE(sp.spPoints,0) 
     AS Puntos_Restantes 
FROM 
     users as u 
    LEFT JOIN 
     (SELECT userid, SUM(points) AS pPoints 
     FROM points 
     WHERE userid = 1 
     GROUP BY userid 
    ) AS p 
    ON u.userid = p.userid 
    LEFT JOIN 
     (SELECT userid, SUM(spPoints) AS spPoints 
     FROM sppoints 
     WHERE userid = 1 
     GROUP BY userid 
    ) AS sp 
    ON u.userid = sp.userid 
WHERE u.userid = 1 ; 

如果你想有一個以上的用戶結果（或所有用戶），更換三WHERE userid=1條件（或完全刪除它們）。

points(userid, points)和sppoints(userid, spPoints)上的索引將有助於提高效率。