问题描述

Create Table #Test (
    ID Int Primary Key Identity,
    Category VarChar(100)
    )

Insert into #Test
(Category)
Values
('Banana'),
('Banana'),
('Banana'),
('Banana'),
('Banana'),
('Banana'),
('Strawberry'),
('Strawberry'),
('Strawberry'),
('Banana'),
('Banana')

Select
     *
    ,ROW_NUMBER() Over (Partition by Category order by ID) as RowNum

From #Test

Order by ID

所以这个脚本返回这个:

So this script returns this:

ID  Category    RowNum
1   Banana      1
2   Banana      2
3   Banana      3
4   Banana      4
5   Banana      5
6   Banana      6
7   Strawberry  1
8   Strawberry  2
9   Strawberry  3
10  Banana      7
11  Banana      8

这是完全有道理的，除了我希望它返回这个:

Which makes perfect sense, except I want it to return this:

ID  Category    RowNum
1   Banana      1
2   Banana      2
3   Banana      3
4   Banana      4
5   Banana      5
6   Banana      6
7   Strawberry  1
8   Strawberry  2
9   Strawberry  3
10  Banana      1
11  Banana      2

我希望它在遇到一组新香蕉时重新开始计数.显然，我的数据并不是真正的香蕉，但它使可视化变得容易.

I want it to restart the count when it hits a new set of Banana. Obviously my data is not really bananas, but it makes it easy to visualize.

香蕉的这种复发被认为是新的，所以当我们看到这一点时，我们想从一个开始数.我一直在绞尽脑汁，想不出一个好的方法来做到这一点.我明白为什么它不工作，但想不出办法让它工作.关于做到这一点的最佳方法有什么建议吗?

This recurrence of bananas is considered to be new, so we want to start counting from one when we see this. I've been racking my brain and can't think of a good way to do this. I understand why it is not working but can't think of a way to make it work. Any advice on the best way to do this?

推荐答案

有几种不同的方法可以解决这个问题.一种方法是 row_number() 方法的不同.此方法将识别相同的相邻类别组:

There are several different ways to approach this. One method is the difference of row_number() approach. This method will identify groups of adjacent categories that are the same:

Select t.*,
       row_number() over (partition by grp, category order by id) as rownum
From (select t.*, 
             (row_number() over (order by id) -
              row_number() over (partition by category order by id)
             ) as grp
      from #Test t
     ) t
Order by ID;

您也可以使用 lag() 计算分组，但这将适用于 SQL Server 2005 和 2008 以及更新的版本.

You can also figure out the groupings using lag(), but this will work in SQL Server 2005 and 2008 as well as more recent versions.

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