Running count for each 2 rows - sql

Question

I am trying to calculate running count for each 2 rows like below,
CREATE TABLE sales
(
EmpId INT,
Yr INT,
Sales DECIMAL(8,2)
)
INSERT INTO sales (EmpId, Yr, Sales)
VALUES (1, 2005, 12000), (1, 2006, 18000), (1, 2007, 25000),
(1, 2008, 25000), (1, 2009, 25000),
(2, 2005, 15000), (2, 2006, 6000), (2, 2007, 6000)
SELECT
EmpId, Yr, sales,
SUM(Sales) OVER (PARTITION BY empid ORDER BY empid ROWS BETWEEN 2 PRECEDING AND CURRENT ROW ) AS TotalSales2
FROM
sales
Output:
EmpId Yr sales TotalSales2
-----------------------------------
1 2005 12000 12000
1 2006 18000 30000
1 2007 25000 55000
1 2008 25000 68000
1 2009 25000 75000
2 2005 15000 15000
2 2006 6000 21000
2 2007 6000 27000
But expected output:
EmpId Yr Sales TotalSales2
-----------------------------------
1 2005 12000 12000
1 2006 18000 30000
1 2007 25000 25000
1 2008 25000 50000
1 2009 25000 25000
2 2005 15000 15000
2 2006 6000 21000
2 2007 6000 6000
What am I doing wrong in this query?
Note: SQL Servre version is 2012.

Answer 1

SELECT EmpId, Yr, Sales,
CASE WHEN ROW_NUMBER() OVER (PARTITION BY EmpId ORDER BY yr) % 2 = 0
THEN sales + lag(sales, 1, 0) OVER (PARTITION BY empid ORDER BY yr)
ELSE sales
END AS TotalSales2
FROM sales
Lag returns the previous row's value - when row_number() is even, add the current row's value to the previous row - otherwise, just show the sales for the current row. Partition each by EmpId, order each by yr - output matches the expected.
Also, thanks so much for adding the DDL/sample data.

Answer 2

The expression:
SUM(Sales) OVER (PARTITION BY empid
ORDER BY empid
ROWS BETWEEN 2 PRECEDING AND CURRENT ROW)
calculates the sum considering the current row and the 2 rows immediately preceding it. So it actually calculates a rolling sum, which is what you really don't want.
I think you are actually looking for something like the following:
;WITH CTE_Group AS (
SELECT EmpId, Yr, sales,
(ROW_NUMBER() OVER (PARTITION BY empid ORDER BY yr) + 1 ) / 2 AS grp
FROM sales
)
SELECT EmpId, Yr, sales,
SUM(sales) OVER (PARTITION BY empid, grp
ORDER BY yr) AS TotalSales2
FROM CTE_Group
The above query uses a CTE in order to calculate field grp: the value of this field is 1 for the first two records of an empid partition, 2 for the next two records, and so on.
Using grp we can calculate the running total of sales for groups of 2 as is the requirement of the OP.
Demo here
Edit:
To offset a larger group of records try using (credit goes to #Max Szczurek for pointing this out):
(ROW_NUMBER() OVER (PARTITION BY empid ORDER BY yr) - 1 ) / n AS grp
where n is the number of records each group contains.

Answer 3

Although answer is already accepted, consider below query also. This will give the required output :
DECLARE #sales TABLE(EmpId INT, Yr INT, Sales DECIMAL(8,2))
INSERT INTO #sales ( EmpId, Yr, Sales )
VALUES (1, 2005, 12000),
(1, 2006, 18000),
(1, 2007, 25000),
(1, 2008, 25000),
(1, 2009, 25000),
(2, 2005, 15000),
(2, 2006, 6000),
(2, 2007, 6000)
;WITH SAMPLE_DATA
AS
(
SELECT ROW_NUMBER()over(partition by empid order by (select 100))SNO,* FROM #Sales
)
SELECT EmpId,Yr,Sales
,CASE WHEN (SNO%2=0)
THEN SALES+
(
SELECT Sales FROM SAMPLE_DATA T2 WHERE T2.EmpId=T1.EmpId AND T2.SNO=T1.SNO-1
)
ELSE Sales END
TotalSales2
FROM SAMPLE_DATA T1
OUTPUT
--------------------------------------
--EmpId Yr Sales TotalSales2
--------------------------------------
1 2005 12000.00 12000.00
1 2006 18000.00 30000.00
1 2007 25000.00 25000.00
1 2008 25000.00 50000.00
1 2009 25000.00 25000.00
2 2005 15000.00 15000.00
2 2006 6000.00 21000.00
2 2007 6000.00 6000.00
--------------------------------------