MYSQL 求和和减法

Question

我有以下SQL查询。

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

我的目标是对 2 个字段求和，然后减去它们，但是当我执行上述查询时，第二个 SUM 是错误的。
表格如下所示：

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

在点数中，我有这个数量：25，在spPoints：10但是当我运行查询时，我得到：

points  spPoints    Puntos_Restantes
 25       30              5

这里出了什么问题？

Answer 1

表users与其他两个表具有一对多关系。这会导致 2 个连接生成一个迷你 Carstesian 积，并在points列中生成具有相同数据的多行 - 然后聚合这些行。

可以使用子查询进行分组，然后联接，以避免此问题：

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 ;

如果要为多个用户（或所有用户）提供结果，请替换三个WHERE userid=1条件（或完全删除它们）。

points(userid, points)和sppoints(userid, spPoints)指数将有助于提高效率。