SqlServer2005连接之前的行

　　Common Table Expressions (CTEs)和Row_number()函数提供了我们一种优雅的解决方案．下面我们来假设一个常见的场景．零售商一张产品(Items)表，还有一张历史价格(PriceHistory)表．数据如下：

产品表

CREATE TABLE [dbo].[Items](
    [ItemId] [int] NOT NULL,
    [Item] [varchar](100) NOT NULL,
 CONSTRAINT [PK_Items] PRIMARY KEY CLUSTERED
(
    [ItemId] ASC
))
GO

 

CREATE TABLE [dbo].[PriceHistory](

    [ItemId] [int] NOT NULL,

    [PriceStartDate] [datetime] NOT NULL,

    [Price] [decimal](10, 2) NOT NULL,

 CONSTRAINT [PK_PriceHistory] PRIMARY KEY CLUSTERED 

(

    [ItemId] ASC,

    [PriceStartDate] ASC

))

GO

ALTER TABLE [dbo].[PriceHistory]  WITH CHECK ADD  CONSTRAINT [FK_PriceHistory_Items] FOREIGN KEY([ItemId])

REFERENCES [dbo].[Items] ([ItemId])

测试数据:

GO
INSERT INTO Items VALUES (1, 'vacuum cleaner')
INSERT INTO Items VALUES (2, 'washing machine')
INSERT INTO Items VALUES (3, 'toothbrush')
INSERT INTO PriceHistory VALUES (1,'2004-03-01',250)
INSERT INTO PriceHistory VALUES (1,'2005-06-15',219.99)
INSERT INTO PriceHistory VALUES (1,'2007-01-03',189.99)
INSERT INTO PriceHistory VALUES (1,'2007-02-03',200.00)
INSERT INTO PriceHistory VALUES (2,'2006-07-12',650.00)
INSERT INTO PriceHistory VALUES (2,'2007-01-03',550.00)
INSERT INTO PriceHistory VALUES (3,'2005-01-01',1.99)
INSERT INTO PriceHistory VALUES (3,'2006-01-01',1.79)
INSERT INTO PriceHistory VALUES (3,'2007-01-01',1.59)
INSERT INTO PriceHistory VALUES (3,'2008-01-01',1.49)

ItemId          PriceStartDate         Price
1    2004-03-01 00:00:00.000    250.00
1    2005-06-15 00:00:00.000    219.99
1    2007-01-03 00:00:00.000    189.99
1    2007-02-03 00:00:00.000    200.00
2    2006-07-12 00:00:00.000    650.00
2    2007-01-03 00:00:00.000    550.00
3    2005-01-01 00:00:00.000    1.99
3    2006-01-01 00:00:00.000    1.79
3    2007-01-01 00:00:00.000    1.59
3    2008-01-01 00:00:00.000    1.49

现在用户希望出一张这样的报表：

Item	Old Price	Range Price	Start Date	End Date
vacuum cleaner		250.00	2004-03-01	2005-06-15
vacuum cleaner	250.00	219.99	2005-06-15	2007-01-03
vacuum cleaner	219.99	189.99	2007-01-03	2007-02-03
vacuum cleaner	189.99	200.00	2007-02-03
washing machine		650.00	2006-07-12	2007-01-03
washing machine	650.00	550.00	2007-01-03
toothbrush		1.99	2005-01-01	2006-01-01
toothbrush	1.99	1.79	2006-01-01	2007-01-01
toothbrush	1.79	1.59	2007-01-01	2008-01-01
toothbrush	1.59	1.49	2008-01-01

第一步，我们首先用CTE来实现，按时间排名产品价格视图：

;WITH PriceCompare AS (
SELECT i.Item, ph.ItemId, ph.PriceStartDate, ph.Price,
ROW_NUMBER() OVER (Partition BY ph.ItemId ORDER BY PriceStartDate) AS rownum
FROM Items i INNER JOIN PriceHistory ph
ON i.ItemId = ph.ItemId)

SELECT * FROM PriceCompare

 

结果是：

Item	ItemId	PriceStartDate	Price	rownum
vacuum cleaner	1	a2004-03-01	250.00	1
vacuum cleaner	1	2005-06-15	219.99	2
vacuum cleaner	1	2007-01-03	189.99	3
vacuum cleaner	1	2007-02-03	200.00	4
washing machine	2	2006-07-12	650.00	1
washing machine	2	2007-01-03	550.00	2
toothbrush	3	2005-01-01	1.99	1
toothbrush	3	2006-01-01	1.79	2
toothbrush	3	2007-01-01	1.59	3
toothbrush	3	2008-01-01	1.49	4

然后增加Select主句：

SELECT currow.Item, prevrow.Price AS OldPrice, currow.Price AS RangePrice, currow.PriceStartDate AS StartDate, nextrow.PriceStartDate AS EndDate
FROM PriceCompare currow
LEFT JOIN PriceCompare nextrow
        ON currow.rownum = nextrow.rownum - 1
        AND currow.ItemId = nextrow.ItemId
LEFT JOIN PriceCompare prevrow
        ON currow.rownum = prevrow.rownum + 1
        AND currow.ItemId = prevrow.ItemId

请注意，我使用的第一行Left Join 为一个项有没有以前的行，就像最后一行没有下一行。

最后我们把写成一个完整的视图吧：

CREATE VIEW [dbo].[PriceCompare] AS
WITH PriceCompare AS
(
SELECT i.Item, ph.ItemId, ph.PriceStartDate, ph.Price,
ROW_NUMBER() OVER (Partition BY ph.ItemId ORDER BY PriceStartDate) AS rownum
FROM
        Items i
INNER JOIN
        PriceHistory ph
ON i.ItemId = ph.ItemId
)

 SELECT
        currow.Item,
        prevrow.Price AS OldPrice,
        currow.Price AS RangePrice,
        currow.PriceStartDate AS StartDate,
        nextrow.PriceStartDate AS EndDate
FROM
        PriceCompare currow
LEFT JOIN PriceCompare nextrow
        ON currow.rownum = nextrow.rownum - 1 AND currow.ItemId = nextrow.ItemId
LEFT JOIN PriceCompare prevrow
        ON currow.rownum = prevrow.rownum + 1 AND currow.ItemId = prevrow.ItemId

实现CTE后让T-SQL如此简洁．这项的场景很多的，例如我们需要存储某一个值历史记录，也可以用这种DB设计方式来实现．
相比传统建立历史表还需要一个结束时间，要少至少一个字段．

希望对您的开发有帮助！

作者：Petter Liu
出处：http://www.cnblogs.com/wintersun/
本文版权归作者和博客园共有，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接，否则保留追究法律责任的权利。
该文章也同时发布在我的独立博客中-Petter Liu Blog。