SQL - Selecting rows with dates before and after column value change - sql

Question

I have a table called test.
In test I have An ID, a value and a date.
The dates are ordered for each ID.
I want to select rows for an ID, before and after a change of value, so the following example table.
RowNum--------ID------- Value -------- Date
1------------------001 ---------1----------- 01/01/2015
2------------------001 ---------1----------- 02/01/2015
3------------------001 ---------1----------- 04/01/2015
4------------------001 ---------1----------- 05/01/2015
5------------------001 ---------1----------- 06/01/2015
6------------------001 ---------1----------- 08/01/2015
7------------------001 ---------0----------- 09/01/2015
8------------------001 ---------0----------- 10/01/2015
9------------------001 ---------0----------- 11/01/2015
10-----------------001 ---------1----------- 12/01/2015
11-----------------001 ---------1----------- 14/01/2015
12------------------002 ---------1----------- 01/01/2015
13------------------002 ---------1----------- 04/01/2015
14------------------002 ---------0----------- 05/01/2015
15------------------002 ---------0----------- 07/01/2015
The result would return rows 6, 7, 9, 10, 13, 14

Answer 1

You could use analytic functions LAG() and LEAD() to access value in preceding and following rows, then check that it does not match value in current row.
SELECT *
FROM (
SELECT RowNum,
ID,
Value,
Date,
LAG(VALUE, 1, VALUE) OVER(ORDER BY RowNum) PrevValue,
LEAD(VALUE, 1, VALUE) OVER(ORDER BY RowNum) NextValue
FROM test)
WHERE PrevValue <> Value
OR NextValue <> Value
Params passed to this functions are
some scalar expression (column name in this case);
offset (1 row before or after);
default value (LAG() will return NULL for first row and LEAD() will return NULL for last row, but they don't seem special in your question, so I used column value as default).

Answer 2

Refer the below one for without using LEAD and LAG:
DECLARE #i INT = 1,
#cnt INT,
#dstvalue INT,
#srcvalue INT
CREATE TABLE #result
(
id INT,
mydate DATE
)
CREATE TABLE #temp1
(
rn INT IDENTITY(1, 1),
id INT,
mydate DATE
)
INSERT INTO #temp1
(id,
mydate)
SELECT id,
mydate
FROM table
ORDER BY id,
mydate
SELECT #cnt = Count(*)
FROM #temp1
SELECT #srcvalue = value
FROM #temp1
WHERE rn = #i
WHILE ( #i <= #cnt )
BEGIN
SELECT #dstvalue = value
FROM #temp1
WHERE rn = #i
IF( #srcvalue = #dstvalue )
BEGIN
SET #i = #i + 1
CONTINUE;
END
ELSE
BEGIN
SET #srcvalue = #dstvalue
INSERT INTO #result
(id,
mydate)
SELECT id,
mydate
FROM #temp
WHERE rn = #i - 1
UNION ALL
SELECT id,
mydate
FROM #temp
WHERE rn = #i
END
SET #i = #i + 1
END
SELECT *
FROM #result

Answer 3

The answer using lag() and lead() is the right answer. If you are using a pre-SQL Server 2012 version, then you can do essentially the same thing using cross apply or a correlated subquery:
select t.*
from test t cross apply
(select top 1 tprev.*
from test tprev
where tprev.date < t.date
order by date desc
) tprev cross apply
(select top 1 tnext.*
from test tnext
where tnext.date > t.date
order by date asc
) tnext
where tprev.value <> tnext.value;