How to reference the current column you are defining using lag? - sql-server

Question

I have a salary table like this:
declare #t table (OrderedID int, EmpID int, EffDate date, Salary money)
insert into #t
values
(1,1234,'20150101',100)
,(2,1234,'20160101',100)
,(3,1234,'20170101',100)
,(4,1234,'20180101',300)
,(1,2351,'20150101',100)
I am trying to get an initial effective date on each row:
First 3 rows have 1/1/2015
4th row has new value 1/1/2018
Here is what I tried with a case and a lag but i can't figure out how to reference the prior value of the column I am creating.
case when OrderedID = 1 then EFFDaTe
when Salary != LAG(Salary,1) then EFFDaTe
else lag(SalaryEFFDT,1) over (order by 1)
end as SalaryEFFDT
Thanks for your help.

Answer 1

As you haven't provided the expected output, I think this is what you want:
declare #t table (OrderedID int, EmpID int, EffDate date, Salary money)
insert into #t
values
(1,1234,'20150101',100)
,(2,1234,'20160101',100)
,(3,1234,'20170101',100)
,(4,1234,'20180101',300)
,(1,2351,'20150101',100)
,(5,1234,'20190101',100)
;with cte as
(Select *, OrderedId - Row_Number() over (partition by EmpId,Salary order by OrderedID) as grp
from #t)
, cte1 as
(Select EmpID, grp, min(effDate) as effDate from cte c group by EmpID, grp)
Select OrderedID, t.EmpID, t.EffDate, t.Salary, c.effDate as computeddate
from cte t join cte1 c on t.EmpID = c.EmpID and t.grp = c.grp
order by OrderedID

Answer 2

So you are trying to get the first effective date for each EmpID? the code below should do that. If that is not your desired output can you put what the output should look like?
declare #t table (OrderedID int, EmpID int, EffDate date, Salary money)
insert into #t
values
(1,1234,'20150101',100)
,(2,1234,'20160101',100)
,(3,1234,'20170101',100)
,(4,1234,'20180101',300)
,(1,2351,'20140101',100)
,(2,2351,'20150101',100)
Select
T.*,FE.FirstEff
From #t T
inner join (Select EmpID,MIN(EffDate) as FirstEff from #t group by
The second set is if you need the first time they have that salary, however you will have issues if someone gets a raise and then a demotion.
Select
T.*,FE.FirstEff
From #t T
inner join (Select EmpID,Salary,MIN(EffDate) as FirstEff from #t group by EmpID,Salary) FE on FE.EmpID = T.EmpID
and FE.Salary = T.Salary