MySQL：多个连接及其对总和的影响

Question

我正在尝试连接多个表并使用SUM命令，当我加入多个表时，我无法使SUM正常工作。 从我在这个主题上所做的（这里和其他地方）的阅读，我相当确定解决方案是在我加入之前求和，但我很难完成这项工作。

如果我运行以下代码，只加入两个表，它可以完美地运行：

SELECT CONCAT(m.nameFirst, " ",m.nameLast) AS Name, sum(b.g) AS G,sum(b.ab) AS AB, sum(b.h) AS H, sum(b.hr) AS HR, sum(b.sb) AS SB, sum(b.bb) AS BB, ROUND(sum(b.h)/sum(b.ab), 3) AS BA, ROUND((sum(b.h)+sum(b.bb)+sum(b.hbp))/(sum(b.ab)+sum(b.bb)+sum(b.hbp)+sum(b.sf)), 3) AS OBP, ROUND((sum(b.h)+sum(b.2b)+2*sum(b.3b)+3*sum(b.hr))/sum(b.ab), 3) as SLG
FROM Master m
JOIN Batting b on m.playerID = b.playerID
WHERE birthMonth = 6 and birthDay = 15
Group by b.playerID
ORDER by b.h DESC

如果我这样做，在连接中添加第三个表，它会破坏总和：

SELECT CONCAT(m.nameFirst, " ",m.nameLast) AS Name, sum(b.g) AS G,sum(b.ab) AS AB, 
[exact same code as above removed for the sake of brevity]
sum(p.W) as WINS
FROM Master m
left JOIN Batting b on m.playerID = b.playerID
left join Pitching p on m.playerID = p.playerID
WHERE birthMonth = 6 and birthDay = 15
Group by m.playerID
ORDER by b.h DESC

正如我所说，我很确定解决方案是在连接之前求和。 我该怎么做呢？

提前致谢。

Answer 1

一种技术是总结表格，然后在这些表格上加入。

SELECT CONCAT(m.nameFirst, " ",m.nameLast) AS Name
    , b.G AS G, b.AB AS AB
    , p.W as WINS
FROM Master m
LEFT JOIN 
  (SELECT playerID, SUM(g) AS G, SUM(ab) AS AB
   FROM Batting
   GROUP BY playerID) AS b
    ON m.playerID = b.playerID
LEFT JOIN 
   (SELECT playerID, sum(W) AS WINS
    FROM Pitching
    GROUP BY playerID) AS p 
  ON m.playerID = p.playerID
WHERE birthMonth = 6 and birthDay = 15

您始终可以将子查询移动到SELECT结构中

SELECT CONCAT(m.nameFirst, " ",m.nameLast) AS Name
    , (SELECT SUM(g)
       FROM Batting AS b
       WHERE m.playerID = b.playerID) AS G
    , (SELECT SUM(ab)
       FROM Batting
       WHERE m.playerID = b.playerID) AS AB
    , (SELECT sum(W)
       FROM Pitching AS p
       WHERE m.playerID = p.playerID) AS WINS
FROM Master m
WHERE birthMonth = 6 and birthDay = 15