SQL Embedded Case - sql

Question

I am new to Sql. The logic of those tables without IF condition always annoys me. I have a table (MyTable) like this:
ID ProjectID ClassType ClassYear Amount
1 1 A 2014 0.00
2 1 A 2014 0.00
3 1 B 2014 300.00
5 1 B 2013 100.00
6 1 C 2015 200.00
7 1 A 2013 0.00
8 1 B 2015 200.00
9 1 B 2014 500.00
11 1 B 2015 230.00
....
My original code as following works fine:
Some code
from
(
Select projectID, sum(currentyear) as CurrentYear, sum(prioryear) as PriorYear, sum(postyear) as PostYear
from
(select ProjectID,
case when Classyear = 2014 then Amount else 0 end as currentyear,
case when ClassYear <2014 then Amount else 0 end as prioryear,
case when ClassYear > 2014 then Amount else 0 end as postyear
from MyTable
where classtype = 'A'
) as subtable
group by subtable.ProjectID
)
Right now I want to add more option for the classtype in the above and can not figure out how. The logic should be:
if total amount from classtype A in a project is 0 (in other word, those three numbers are all 0)then the where condition should change to classtype = 'B'. If classtype B are still all 0 then use the classtype C. (It will be OK if C are still 0).
Where should I put the new case condition?

Answer 1

we can get total using GROUP BY for each class type and then filter appropriate value.
SELECT *
FROM
(
select *, ROW_NUMBER() over ( partition by projectId order by classType) as seq FROM
(
Select projectID, classType, sum(Amount) as Total,
sum(case when Classyear = 2014 then Amount else 0 end) as CurrentYear,
sum(case when ClassYear <2014 then Amount else 0 end) as PriorYear,
sum( case when ClassYear > 2014 then Amount else 0 end) as PostYear
from
myTable
group by ProjectID, classType
) T
Where (T.Total >0 and classType <> 'C') or classType ='C'
) C
where seq =1