Finding monthly changes in a data set

Question

Hello
Given the below sample table and data, I am after a SQL script that can generate the sample output shown.
In a nutshell, I need to identify in a derived column, compared to the previous month, what's:
1) Not changed (e.g. 'A' in Feb)
2) Been removed (e.g. 'B' in Feb)
3) Been added (e.g. 'D' in Feb)
Can someone help with this? From what I remember Windows function would be ideal for this but it has been a long time since I used them...
CREATE TABLE [dbo].[Data](
[MyDate] [date] NOT NULL,
[MyText] [varchar](10) NOT NULL
) ON [PRIMARY]
GO
-- Jan
INSERT INTO [dbo].[Data](MyDate,MyText) VALUES ('01Jan2014', 'A')
INSERT INTO [dbo].[Data](MyDate,MyText) VALUES ('01Jan2014', 'B')
INSERT INTO [dbo].[Data](MyDate,MyText) VALUES ('01Jan2014', 'C')
-- Feb - 'B' dropped, 'D' added
INSERT INTO [dbo].[Data](MyDate,MyText) VALUES ('01Feb2014', 'A')
INSERT INTO [dbo].[Data](MyDate,MyText) VALUES ('01Feb2014', 'C')
INSERT INTO [dbo].[Data](MyDate,MyText) VALUES ('01Feb2014', 'D')
-- Mar - 'A' and 'C' dropped, 'E' and 'F' added
INSERT INTO [dbo].[Data](MyDate,MyText) VALUES ('01Mar2014', 'D')
INSERT INTO [dbo].[Data](MyDate,MyText) VALUES ('01Mar2014', 'E')
INSERT INTO [dbo].[Data](MyDate,MyText) VALUES ('01Mar2014', 'F')
/* Sample required output
MyDate MyText State
2014-01-01 A 0
2014-01-01 B 0
2014-01-01 C 0
2014-02-01 A 0
2014-01-01 B -1
2014-02-01 C 0
2014-02-01 D 1
2014-02-01 A -1
2014-02-01 C -1
2014-03-01 D 0
2014-03-01 E 1
2014-03-01 F 1

Answer 1

;With CTE
AS
SELECT d.MyDate,t.MyText,d1.MyDate AS RetDate,ROW_NUMBER() OVER (PARTITION BY t.MyText ORDER BY d.MyDate) AS Rn
FROM (SELECT DISTINCT MyDate FROM Data)d
INNER JOIN (SELECT DISTINCT MyText,MIN(Mydate) AS MinDate,MAX(Mydate) AS MaxDate
FROM Data
GROUP BY MyText
)t
ON t.MinDate <= d.MyDate
AND d.MyDate <= DATEADD(mm,1,t.MaxDate )
LEFT JOIN Data d1
ON d1.MyDate = d.MyDate
AND d1.MyText = t.MyText
SELECT MyDate,MyText,
CASE WHEN RetDate IS NULL THEN -1
WHEN Rn > 1 THEN 0
ELSE 1
END
FROM CTE
ORDER BY MyDate,MyText
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