How to determine an Increase in Employee Salary from consecutive Contract Rows? - sql

Question

I got a problem in my query :
My table store data like this
ContractID | Staff_ID | EffectDate | End Date | Salary | active
-------------------------------------------------------------------------
1 | 1 | 2013-01-01 | 2013-12-30 | 100 | 0
2 | 1 | 2014-01-01 | 2014-12-30 | 150 | 0
3 | 1 | 2015-01-01 | 2015-12-30 | 200 | 1
4 | 2 | 2014-05-01 | 2015-04-30 | 500 | 0
5 | 2 | 2015-05-01 | 2016-04-30 | 700 | 1
I would like to write a query like below:
ContractID | Staff_ID | EffectDate | End Date | Salary | Increase
-------------------------------------------------------------------------
1 | 1 | 2013-01-01 | 2013-12-30 | 100 | 0
2 | 1 | 2014-01-01 | 2014-12-30 | 150 | 50
3 | 1 | 2015-01-01 | 2015-12-30 | 200 | 50
4 | 2 | 2014-05-01 | 2015-04-30 | 500 | 0
5 | 2 | 2015-05-01 | 2016-04-30 | 700 | 200
-------------------------------------------------------------------------
Increase column is calculated by current contract minus previous contract
I use sql server 2008 R2

Answer 1

Unfortunately 2008R2 doesn't have access to LAG, but you can simulate the effect of obtaining the previous row (prev) in the scope of a current row (cur), with a RANKing and a self join to the previous ranked row, in the same partition by Staff_ID):
With CTE AS
(
SELECT [ContractID], [Staff_ID], [EffectDate], [End Date], [Salary],[active],
ROW_NUMBER() OVER (Partition BY Staff_ID ORDER BY ContractID) AS Rnk
FROM Table1
)
SELECT cur.[ContractID], cur.[Staff_ID], cur.[EffectDate], cur.[End Date],
cur.[Salary], cur.Rnk,
CASE WHEN (cur.Rnk = 1) THEN 0 -- i.e. baseline salary
ELSE cur.Salary - prev.Salary END AS Increase
FROM CTE cur
LEFT OUTER JOIN CTE prev
ON cur.[Staff_ID] = prev.Staff_ID and cur.Rnk - 1 = prev.Rnk;
(If ContractId is always perfectly incrementing, we wouldn't need the ROW_NUMBER and could join on incrementing ContractIds, I didn't want to make this assumption).
SqlFiddle here
Edit
If you have Sql 2012 and later, the LEAD and LAG Analytic Functions make this kind of query much simpler:
SELECT [ContractID], [Staff_ID], [EffectDate], [End Date], [Salary],
Salary - LAG(Salary, 1, Salary) OVER (Partition BY Staff_ID ORDER BY ContractID) AS Incr
FROM Table1
Updated SqlFiddle
One trick here is that we are calculating delta increments in salary, so for the first employee contract we need to return the current salary so that Salary - Salary = 0 for the first increase.