Question

我有六个表 - 项目，设备，鱼，员工和联结表 - Project_Equipment，Project_Fish和Project_Staff.I想要检索项目总成本。
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，我得到了两倍正确金额的价格。

Answer 1

您的查询最终会出现大量重复结果。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，我们来看一下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

这是预期的;每个项目使用的每件设备的价格和数量清单。但是，当我们INNER JOIN 到project_fish时，您认为会发生什么？第一个结果是项目1在其中两次，项目2在其中两次，因此我们最终得到该结果的每个组合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

此重复将继续每个内部联接。您的价格金额将不会总是2倍，实际上取决于所有联接的组合数量。

所以，你不能真正做你想对这个特定查询做的事情。相反，你必须分别计算每个关系的成本。然后你将所有这些加在一起。您可以单独选择每个成本并将成本计算到ProjectID和ProjectCost列，然后使用UNION完全连接这些成本，然后再按{{1}对结果进行分组并总结各个ProjectID小计。

我解释得很糟糕，但把它想象为设备，鱼和工资成本的小计，然后将所有小计分成一个表并总结那个。 E.g：

ProjectCost

每个子查询都会生成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列和projectid列。单独运行子查询（在parens之间）以查看结果。外部查询然后添加项目的小计。

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

使用一个主表连接多个

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