Tsql Case Statement

Hi,
Here is the scenario. I want to add last year sale dollars in accordance with current period in exsiting fact table.
And below is the syntax.

Syntax:
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
select
a. store_key,
a.fisc_date_key,
sum(a.net_sale_Dollars) as sale_TY ,
sum ( b.net_sale_dollars ) as sale_LY ,
a.division_name,
a.department_number

fromFact 1 as a

,Fact 1 as b

Whereb.fisc_date_key = (a.fisc_date_key -364)
and a.division_name=b.division_name
and a.department_number =b.department_number
and a.store_key = b.store_key

group by
a.division_name,
a.department_number,
a.fisc_date_key,
a.store_key

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------


The current table from this query is showing like:

store_key date_key sale_TY sales_LY div dept
------------------------------------------------------------------------------------------------
1 1 30 20 ABC 1
2 1 20 20 ABC 3

But, if we assume that in the current date, dept = 2 has a sale amount, and in parallel year if dept=2 does not have any sale then this information was excluded.

The structure of table that I want to create must look like:


store_key date_key sale_TY sales_LY div dept
------------------------------------------------------------------------------------------------
1 1 30 20 ABC 1
2 1 20 20 ABC 3
2 1 15 0 ABC 2

>>>> want to put 0 value where only one side ( current or parrallel period) has sales info.

So, I'm thinking the case statement like:


Case statement logic like:
------------------------------------------------------------
if a. dept not exist in b.dept
then Sale TY -> a.net_sale_dollars
Sale LY -> 0

if b.dept not exist in a.dept
then sale TY -> 0
sale LY -> b. net_sale_dollars
-------------------------------------------------------------

below is the syntax which doesn't work (it's wrong):

Syntax:
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

select
a. store_key,
a.fisc_date_key,
sum(case when a.department_number = b.department_number then a.net_sale_dollars
else case when a.department_number NOT IN (b.department_number)then a.net_sale_dollars else null end)
as sale_TY ,
sum ( case when b.department_number =a.department_number then b.net_sale_dollars

else case when a.department_number NOT IN (b.department_number) as sale_LY,
a.division_name,
a.department_number

fromFact 1 as a

Fact 1 as b

Whereb.fisc_date_key = (a.fisc_date_key -364)
and a.division_name=b.division_name
and a.department_number =b.department_number
and a.store_key = b.store_key

group by
a.division_name,
a.department_number,
a.fisc_date_key,
a.store_key

,
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------


Is it possible to create these kind of structure?
Please give me some comments.
Thanks.