SUM Mixed With Case In Select Statement I am attempting to run this SELECT statement, but I keep getting error -


  Msg 130, Level 15, State 1, Line 9
  Cannot perform an aggregate function on an expression containing an aggregate or a subquery.


This is my syntax, how should this be re-written in order to succesfully execute?  I also add here make-up DDL to better illustrate question.

Declare @info TABLE (region varchar(50), p1 float, p2 float, p3 float, p4   float, p5 float, necombs float, d1 datetime)
INSERT INTO @info (region, p1, p2, p3, p4, p5, necombs, d1) VALUES
('west', 1.11, 2.22, 3.33, 4.44, 5.11, 22.00, '01/01/2015 08:00:00.000')
,('east', 2.11, 3.22, 3.33, 1.44, 2.11, 12.00, '01/03/2016 08:00:00.000')
,('north', 1.11, 2.22, 3.33, 4.44, 5.11, 22.00, '01/05/2015 08:00:00.000')

Select
region
,SUM(CASE WHEN YEAR(d1) = '2015' THEN ISNULL(p1, 0)+ISNULL(p2, 0)+ISNULL(p3, 0)+ISNULL(p4, 0)+ISNULL(p5, 0)/NULLIF(SUM(ISNULL(necombs, 0)), 0) ELSE 0 END) [Answer]
FROM @info
Group By Region
The accepted answer doesn't show the 2016 row, which I would expect to be 0 based on the initial SQL in the question.

Here's an alternative that would give the additional 2016 row:

 SELECT i.region,
        DATEPART(YEAR, i.d1) AS YearNumber,
        SUM(rd.Calc) / ISNULL(SUM(ISNULL(necombs, 0)), 0) Answer
 FROM @info i
 CROSS APPLY (VALUES(CASE WHEN DATEPART(YEAR, d1) = '2015' THEN ISNULL(p1, 0) + ISNULL(p2, 0) + ISNULL(p3, 0) + ISNULL(p4, 0) + ISNULL(p5, 0) ELSE 0 END)) AS rd(Calc)
 GROUP BY Region, DATEPART(YEAR, i.d1);