Question

我有3张桌子：

Trip  Promotion  Promotion Cost.
  1 ---- M   1  --------- M

示例数据包括：

TripID    TripName    Date
XYZ123     Hawaii    09/06/09
YTU574     Japan     09/09/09
GHR752     US        11/07/09


PromotionID    TripID    Name
1              XYZ123    Poster
2              XYZ123    Brochure
3              GHR752    TV ad

CostID    PromotionID   Cost
  1           1         $50
  2           1         $100
  3           1         $120
  4           3         $2000
  5           2         $500

我正在尝试构建这样的查询：

TripID     Number of Promotions     Total Cost
XYZ123             2                  $770
GHR752             1                  $2000

我拥有的是：

SELECT
      Trip.TripID, Count(Trip.TripID) AS [Number Of Promotions], Sum(PromotionCost.Cost) AS SumOfCost
FROM
     Trip
     INNER JOIN
         (Promotion
          INNER JOIN
          PromotionCost ON Promotion.PromotionID = PromotionCost.PromotionID
         ) ON Trip.TripID = Promotion.TripID
GROUP BY
       Trip.TripID;

它给了我这样的东西：

TripID     Number of Promotions     Total Cost
XYZ123             4                  $770
GHR752             1                  $2000

我不确定为什么促销数量会像第一个（XYZ123）那样混乱。似乎JOIN以某种方式影响它，因为如果我使用它：

SELECT
      Trip.TripID, Count(Trip.TripID) AS [Number Of Promotions], 
FROM
      Trip
      INNER JOIN
      Promotion ON Trip.TripID = Promotion.TripID
GROUP BY
      Trip.TripID;

它为我提供了正确的促销数量，仅为2个。

Answer 1

您可以在子查询中累计每个促销的费用。这样，每次促销只获得一行，COUNT用于计算每次促销的促销数量。例如：

select 
    t.TripId
,   count(p.PromotionId) as [Number of Promotions]
,   sum(pc.PromotionCost) as [Total Cost]
from trip t
left join promotions p on p.TripId = t.TripId
left join (
    select 
        PromotionId
    ,   PromotionCost = sum(cost)
    from Promotions
    group by PromotionId
) pc on pc.PromotionId = p.PromotionId
group by t.TripId

如果MS Access不允许子查询，您可以将子查询存储在视图中，然后加入该查询。

Answer 2

您可以尝试使用COUNT(DISTINCT)

来补偿重复的促销行

SELECT Trip.TripID, Count(DISTINCT Promotion.PromotionID) AS [Number Of Promotions],
  Sum(PromotionCost.Cost) AS SumOfCost
FROM Trip INNER JOIN Promotion ON Trip.TripID = Promotion.TripID
INNER JOIN PromotionCost ON Promotion.PromotionID = PromotionCost.PromotionID
GROUP BY Trip.TripID;

正在发生的事情是，默认情况下，COUNT()计算所有连接完成后生成的行数。 TripID XYZ123有四个促销费用，因此有四行，即使TripId在这四行中多次出现。

如果您在没有GROUP BY的情况下尝试类似的查询，则更容易可视化：

SELECT Trip.TripID, Promotion.PromotionID, PromotionCost.Cost
FROM Trip INNER JOIN Promotion ON Trip.TripID = Promotion.TripID
INNER JOIN PromotionCost ON Promotion.PromotionID = PromotionCost.PromotionID;

您将看到XYZ123的四行（具有重复的PromotionID值）和GHR752的一行。

重新评论MS Access不支持COUNT(DISTINCT)：如果是这种情况，则不应在单个查询中执行此操作。在两个查询中执行：

SELECT Trip.TripID, SUM(PromotionCost.Cost) AS SumOfCost
FROM Trip INNER JOIN Promotion ON Trip.TripID = Promotion.TripID
INNER JOIN PromotionCost ON Promotion.PromotionID = PromotionCost.PromotionID
GROUP BY Trip.TripID;

SELECT Trip.TripID, Count(Promotion.PromotionID) AS [Number Of Promotions]
FROM Trip INNER JOIN Promotion ON Trip.TripID = Promotion.TripID
GROUP BY Trip.TripID;

替代方案是一个使用子查询的非常复杂的解决方案，在Microsoft的这篇文章中有描述：

http://blogs.msdn.com/access/archive/2007/09/19/writing-a-count-distinct-query-in-access.aspx

Answer 3

不是您的问题的答案，而是一个有用的建议（我希望）：使用SQL Server Management Studio的可视化设计器将您的查询转换为视图，并检查生成的SQL代码。您不必实际保留和使用生成的视图，但这是一个通过示例学习的好方法。每当我遇到复杂的查询时，我就会这样做。

修改即可。惭愧我，我不读标签：问题是MS-Access相关，而不是SQL Server相关。无论如何，我认为我的建议仍然有效，因为概念学习是关注点，因为SQL语法是相似的。

我怎样才能加入这三张桌子

3 个答案: