本文介绍了sql仅按顺序分组的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
假设我有下表:
MyTable
---------
| 1 | A |
| 2 | A |
| 3 | A |
| 4 | B |
| 5 | B |
| 6 | B |
| 7 | A |
| 8 | A |
---------
我需要 sql 查询输出以下内容:
I need the sql query to output the following:
---------
| 3 | A |
| 3 | B |
| 2 | A |
---------
基本上我正在做一个 group by 但只针对在序列中在一起的行.有什么想法吗?
Basically I'm doing a group by but only for rows which are together in the sequence. Any ideas?
请注意，数据库位于 sql server 2008 上.有一篇关于此主题的帖子，但它使用了 oracle 的 lag() 函数.
Note that the database is on sql server 2008. There is a post on this topic however it uses oracle's lag() function.
推荐答案
这被称为孤岛"问题.使用 Itzik Ben Gan 的方法:
This is known as the "islands" problem. Using Itzik Ben Gan's approach:
;WITH YourTable AS
(
SELECT 1 AS N, 'A' AS C UNION ALL
SELECT 2 AS N, 'A' AS C UNION ALL
SELECT 3 AS N, 'A' AS C UNION ALL
SELECT 4 AS N, 'B' AS C UNION ALL
SELECT 5 AS N, 'B' AS C UNION ALL
SELECT 6 AS N, 'B' AS C UNION ALL
SELECT 7 AS N, 'A' AS C UNION ALL
SELECT 8 AS N, 'A' AS C
),
T
AS (SELECT N,
C,
DENSE_RANK() OVER (ORDER BY N) -
DENSE_RANK() OVER (PARTITION BY C ORDER BY N) AS Grp
FROM YourTable)
SELECT COUNT(*),
C
FROM T
GROUP BY C,
Grp
ORDER BY MIN(N)
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