create table saltst
(
empid int,
saldate datetime,
salary int
)

--Insert Records For Each Employee
Insert into saltst values(1, '2014-01-01', 1000)
Insert into saltst values(1, '2014-03-26', 3400)

Insert into saltst values(2, '2014-01-25', 2000)
Insert into saltst values(2, '2014-06-11', 3000)
Insert into saltst values(2, '2014-09-01', 4000)

Insert into saltst values(3, '2013-01-01', 4000)
Insert into saltst values(3, '2014-01-01', 1000)

Insert into saltst values(4, '2014-11-01', 3500)

-- Run Select
select * from saltst

-- Add DENSE_RANK()
Select empid, saldate, salary,
DENSE_RANK() OVER(partition by empid order by saldate) as dr
From saltst

Go

-- Solution - Substract Last Salary From Previous Salary
with SalaryRank as (
select empid, saldate, salary,
DENSE_RANK() OVER(partition by empid order by saldate) as dr
from saltst),

Currentsal as
(select empid, salary, saldate from SalaryRank a 
where dr in(select max(dr) from SalaryRank b where a.empid=b.empid)),

PreviousSal as 
(select empid, salary, saldate from SalaryRank a 
where dr in(select case when max(dr) = 1 then 1 else max(dr) -1 end mx from SalaryRank b where a.empid=b.empid))

--select * from SalaryRank;
--select * from Currentsal;
--select * From PreviousSal;

select a.empid, a.salary as Currentsal, b.salary as PreviousSal, a.salary - b.salary as SalDiff,
	   a.saldate As CurrentSalDate, b.saldate As PreviousSalDate
from
(select * from Currentsal) a
 join
(select * from PreviousSal) b
on a.empid = b.empid