访问 - 如何计算订单之间的平均天数（无游标）

Question

我想在访问中创建一个查询，返回订单之间的平均天数。 表结构是： 表名：

Orders 

表列：

idOrder (integer) 
idClient (integer) 
OrderDate (date in format dd/mm/yyyy) 
OrderAmount (money) 

一个简单的查询表会返回此：

1 1131 01/01/2015 $235 
2 1145 02/01/2015 $11 
3 1131 03/01/2015 $256 
4 1131 04/01/2015 $300 
5 1146 05/01/2015 $25 
6 1145 10/01/2015 $15 
7 1145 20/01/2015 $4 
8 1131 21/01/2015 $45 

在此

例如，

client 1131 has orders on the days: 01/01/2015; 03/01/2015; 04/01/2015; 21/01/2015 
client 1145 has orders on the days: 02/01/2015; 10/01/2015; 20/01/2015 
client 1146 has orders on days: 05/01/2015 

我想创建一个查询，订单之间返回每个客户的平均天数：

1131 5 
1145 6 
1146 null 

如果这将是SQL Server时，我会用一个光标，但在Access中，我不知道怎么做。

任何帮助？

Answer 1

你不需要SQL Server中的游标。 话虽如此，请尝试下面的查询。 如果客户下了一个订单，逻辑上结果应该是0（零），而不是空。 空值应反映任何没有任何订单的客户。

SELECT Orders.idClient, (Max(Orders.OrderDate)-Min(Orders.OrderDate))/Count(*) AS [Days between orders] 
FROM Orders 
GROUP BY Orders.idClient 

Answer 2

建立这样的查询：

Select 
    idClient, 
    Avg(DaysPast) As AverageOrderDays 
From  
    (Select 
     idClient, 
     DateDiff("d", 
       (Select Max(T.[Purchase Week]) 
       From Orders As T 
       Where T.idClient=Orders.idClient 
       And T.[Purchase Week] < Orders.[Purchase Week]), 
      [Purchase Week]) AS DaysPast 
    From 
     Orders) As OrderDays 
Group By 
    idClient