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我有六个表 - Project、Equipment、Fish、Staff 和连接表 - Project_Equipment、Project_Fish 和 Project_Staff。我想检索项目总成本。
所以，我写了如下声明， 

SELECT P.ProjectID, (SUM(E.EquipPrice*PE.EQuantity)+SUM(F.FishPrice*PF.FQuantity)+SUM(PS.Salary)) as ProjectCost  
FROM Equipment as E INNER JOIN Project_Equipment as PE  
ON E.EquipID=PE.EquipID  
INNER JOIN Project as P  
ON PE.ProjectID=P.ProjectID  
INNER JOIN Project_Fish as PF  
ON P.ProjectID=PF.ProjectID  
INNER JOIN Fish as F  
ON PF.FishID=F.FishID  
INNER JOIN Project_Staff as PS  
ON P.ProjectID=PS.ProjectID  
INNER JOIN Staff as S
PS.StaffID=S.StaffID  
GROUP BY ProjectID

但是，我得到了两倍正确数量的价格。

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1 回答 1

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您的查询最终会得到很多重复的结果。考虑以下更简单的情况：

Table: Project
ProjectID  EQuantity
  1          1
  2          1

Table: Equipment
EquipID    EPrice
  1          1
  2          1

Table: Fish
FishID
  1
  2

Table: Project_Equipment
ProjectID  EquipID
  1          1
  1          2
  2          1
  2          2

Table: Project_Fish
ProjectID  FishID
  1          1 
  1          2
  2          1
  2          2

现在，让我们只看一个Project_Equipment查询的结果：

SELECT p.projectid, e.eprice, pe.equantity FROM project p
INNER JOIN project_equipment pe ON pe.projectid=p.projectid
INNER JOIN equipment e ON e.equipid=pe.equipid

ProjectID EPrice EQuantity
1    1    1   // a
1    1    1   // b
2    1    1   // c
2    1    1   // d

正如预期的那样；每个项目使用的每件设备的价格和数量清单。但是你认为当我们这样做时会INNER JOIN 发生什么project_fish？第一个结果中有两次项目 1 和两次项目 2，所以我们最终得到了该结果和project_fish!

SELECT p.projectid, e.eprice, pe.equantity, f.fishid FROM project p
INNER JOIN project_equipment pe ON pe.projectid=p.projectid
INNER JOIN equipment e ON e.equipid=pe.equipid
INNER JOIN project_fish pf ON pf.projectid=p.projectid

ProjectID EPrice EQuantity FishID
1    1    1    1   // from a above
1    1    1    2   // from a above
1    1    1    1   // from b above 
1    1    1    2   // from b above
2    1    1    1   // from c above
2    1    1    2   // from c above
2    1    1    1   // from d above
2    1    1    2   // from d above

这种复制将在每个内部连接中继续。您的价格将减少的金额并不总是 2 倍，它实际上取决于您所有联接的组合数量。

所以，你不能真正用这个特定的查询来做你想做的事情。相反，您必须分别计算每个关系的成本。然后你把所有这些加在一起。您可以通过分别选择每一个并将成本计算到一个ProjectID和ProjectCost列中，使用UNION将它们完全连接起来，然后再次对结果进行分组ProjectID并对各个ProjectCost小计求和来做到这一点。

我解释得很糟糕，但可以将其视为对设备、鱼和工资成本的小计，然后将所有这些小计汇总到一张表中并将其相加。例如：

SELECT x.projectid, SUM(x.ProjectCost) FROM 
(
    SELECT p.projectid, SUM(e.eprice * pe.equantity) ProjectCost FROM project p
    INNER JOIN project_equipment pe ON pe.projectid=p.projectid
    INNER JOIN equipment e ON e.equipid=pe.equipid
    GROUP BY p.projectid
  UNION
    SELECT p.projectid, SUM(f.fprice * pf.fquantity) ProjectCost FROM project p
    INNER JOIN project_fish pf ON pf.projectid=p.projectid
    INNER JOIN fish f ON f.fishid=pf.fishid
    GROUP BY p.projectid
  UNION
    SELECT p.projectid, SUM(s.salary) ProjectCost FROM project p
    INNER JOIN project_staff ps ON ps.staffid=p.projectid
    INNER JOIN staff s ON s.staffid=ps.staffid
    GROUP BY p.projectid
) x
GROUP BY x.projectid

每个子查询产生一projectid列和一ProjectCost列。单独运行子查询（在括号之间）以查看结果。然后外部查询添加项目的小计。

抱歉，顺便说一句，我在测试时将您的 EquipPrice 和 FishPrice 列重命名为 EPrice 和 FPrice。

于 2013-08-14T01:58:00.560 回答