SQL Server - 计算数据集中属性更改的次数(非并发)

时间:2011-02-17 08:57:11

标签: sql sql-server tsql subquery gaps-and-islands

我有一个查询,根据事件是否在给定日期发生,返回1或0。这是按日期排序的。基本上,一个简单的结果集是:

Date         |  Type
---------------------
2010-09-27      1
2010-10-11      1
2010-11-29      0
2010-12-06      0
2010-12-13      1
2010-12-15      0
2010-12-17      0
2011-01-03      1
2011-01-04      0

我现在希望能够做的是计算单独的,非并发的'0'实例的数量 - 即计算有多少不同的0组出现。

在上面的例子中,答案应该是3(1组2,然后另一组2,最后1结束)。

希望上面的例子说明了我想要达到的目标。我一直在寻找,但我发现很难简洁地描述我在寻找什么,因此没有找到任何相关的东西。

提前致谢,

约什

3 个答案:

答案 0 :(得分:3)

您可以在CTE中为每一行提供一个数字。然后,您可以将表连接到自身以查找上一行。知道前一行,您可以将前一行的数量加1,当前行为0.例如:

; with NumberedRows as
        (
        select  row_number() over (order by date) as rn
        ,       type
        from    YourTable
        )
select  sum(case when cur.type = 0 and IsNull(prev.type,1) = 1 then 1 end)
from    NumberedRows cur
left join    
        NumberedRows prev
on      cur.rn = prev.rn + 1

答案 1 :(得分:2)

这是“岛屿”问题的变体。我的第一个答案使用Itzik Ben Gan的双row_number技巧来有效地识别连续的数据组。 Type,Grp的组合标识数据中的每个岛屿。

You can read more about the different approaches to tackling this problem here.

;WITH T AS (
    SELECT  *,
            ROW_NUMBER() OVER(ORDER BY Date) -
            ROW_NUMBER() OVER(PARTITION BY Type ORDER BY Date)  AS Grp
    FROM    YourTable
)
SELECT  COUNT(DISTINCT Grp)
FROM    T
WHERE Type=0

我的第二个答案需要单次传递数据。它不能保证工作,但其原理与许多人成功用于连接字符串而没有问题的技术相同。

DECLARE @Count int = 0

SELECT @Count = CASE WHEN Type = 0 AND @Count <=0 THEN -@Count+1 
                     WHEN Type = 1 AND @Count > 0 THEN - @Count
                     ELSE @Count END

FROM YourTable
ORDER BY Date

SELECT ABS(@Count)

答案 2 :(得分:1)

使用Sql Server 2005 +

查看此示例
DECLARE @Table TABLE(
        Date DATETIME,
        Type INT
)

INSERT INTO @Table SELECT '2010-09-27',1
INSERT INTO @Table SELECT '2010-10-11',1
INSERT INTO @Table SELECT '2010-11-29',0
INSERT INTO @Table SELECT '2010-12-06',0
INSERT INTO @Table SELECT '2010-12-13',1
INSERT INTO @Table SELECT '2010-12-15',0
INSERT INTO @Table SELECT '2010-12-17',0
INSERT INTO @Table SELECT '2011-01-03',1
INSERT INTO @Table SELECT '2011-01-04',0

;WITH Vals AS (
    SELECT  *,
            ROW_NUMBER() OVER(ORDER BY Date) ROWID
    FROM    @Table
)
SELECT  v.*
FROM    Vals v LEFT JOIN
        Vals vNext ON   v.ROWID + 1 = vNext.ROWID
WHERE   v.Type = 0
AND     (vNext.Type = 1 OR vNext.Type IS NULL)