sql – 多个连接语句未返回预期结果

2023年12月21日 176次阅读

我必须编写一个具有以下要求的查询：

The query should return a list of all
entry values for a customer named
“Steve” and for each date shown (if
available) the most recent status detail for
that date.

客户表

CustomerID | CustomerName
1          |    Steve
2          |    John

条目表

CustomerID | EntryDate | EntryValue 
 1         | 5/4/2010  |    200.0  
 1         | 4/4/2010  |    100.0  
 1         | 3/4/2010  |    150.0
 1         | 2/4/2010  |    170.0
 2         | 5/4/2010  |    220.0

状态表

CustomerID | StatusDate | Detail
1          | 5/28/2010  | D
1          | 4/24/2010  | S
1          | 4/5/2010   | P
1          | 2/28/2010  | A

预期的产出是：

CustomerName |   Date    |   OrderCost |   Detail 
  Steve      | 5/4/2010  |    200.0    |  S
  Steve      | 4/4/2010  |    100.0    |  A
  Steve      | 3/4/2010  |    75.0     |  A
  Steve      | 3/4/2010  |    75.0     |  <null>

我认为预期的输出可能是错误的,它应该是：

CustomerName |   Date    |   OrderCost |   Detail 
  Steve      | 5/4/2010  |    200.0    |  S
  Steve      | 4/4/2010  |    100.0    |  A
  Steve      | 3/4/2010  |    150.0     |  A
  Steve      | 2/4/2010  |    170.0     |  <null>

鉴于这个要求,我不明白为什么2010年3月4日的日期会发生两次,第二次会有一个细节.我写了以下查询：

我写了以下查询：

SELECT  Customers.CustomerName, Entries.EntryDate, Entries.EntryValue, Status.Detail
FROM   Customers
    INNER JOIN Entries ON Customers.CustomerID = Entries.CustomerID
    LEFT OUTER JOIN Status ON Status.CustomerID = Customers.CustomersID AND Status.StatusDate <= Entries.EntryDate
WHERE (Customers.CustomerName = 'Steve')

我的查询结果如下：

CustomerName| EntryDate |   EntryValue |   Detail
Steve      |  5/4/2010 |    200.00  |   S
Steve      |  5/4/2010 |    200.00  |   P
Steve      |  5/4/2010 |    200.00  |   A
Steve      |  4/4/2010 |    100.00  |   A
Steve      |  3/4/2010 |    150.00  |   A
Steve      |  2/4/2010 |    170.00  |   NULL

我在这里做错了什么提示？我无法弄明白……

更新
我已将订单更改为条目,因此不会让我们感到困惑.

最佳答案 您获得的结果比预期的要多,因为状态表中的许多行都满足第二个JOIN条件(例如,有5个状态日期早于5/4,因此此日期在结果集中显示3次).

您需要JOIN状态表,但只能获得一个匹配(最新).这可以通过几种方式完成,AFAIK通常带有子查询.我认为你的情况相当复杂 – 我使用临时表.希望它有帮助…(我目前没有数据库来测试它,希望没有愚蠢的语法错误).

DROP TABLE IF EXISTS temp;

CREATE TABLE temp AS  -- This temp table is basically the result set you got
 (SELECT c.CustomerName, e.EntryDate, e.EntryValue, s.Detail, s.StatusDate
  FROM Customers c
    INNER JOIN Entires e ON c.CustomerID = e.CustomerID
    LEFT OUTER JOIN Status s ON s.CustomerID = c.CustomersID
                    AND s.StatusDate <= e.EntryDate
  WHERE (c.CustomerName = 'Steve')
 );

SELECT t.CustomerName, t.EntryDate, t.EntryValue, t.Detail
FROM temp t
WHERE t.StatusDate = (SELECT MAX(t2.StatusDate) 
                      FROM temp t2 
                      WHERE t2.EntryDate = t.EntryDate);

为了避免创建临时表,我相信这会起作用(请尝试让我知道！)

SELECT t.CustomerName, t.EntryDate, t.EntryValue, t.Detail
FROM (SELECT c.CustomerName, e.EntryDate, e.EntryValue, s.Detail, s.StatusDate
      FROM Customers c
      INNER JOIN Entries e ON c.CustomerID = e.CustomerID
      LEFT OUTER JOIN Status s ON s.CustomerID = c.CustomersID
                      AND s.StatusDate <= e.EntryDate
      WHERE c.CustomerName = 'Steve') AS t
WHERE t.StatusDate = (SELECT MAX(t2.StatusDate)
                      FROM temp t2
                      WHERE t2.EntryDate = t.EntryDate);
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