'Can not perform an aggregate function on an aggregate or a subquery' error on SQL Server

Question

Select A.nEvrakKalemID As ID, 
  A.cEvrakKalem As [Inventory Name],
  SUM((IsNull((Select SUM(B.nMiktar) Where B.nEvrakTip = 3 or B.nEvrakTip = 7),0)) - (IsNull((Select SUM(B.nMiktar) Where B.nEvrakTip = 4 or B.nEvrakTip = 8),0))) As Inventory
From MasrafAnaliz.dbo.EvrakKalem A 
Left Outer Join MasrafAnaliz.dbo.EvrakKalemHareket B
On B.nKodEvrakKalem = A.nEvrakKalemID
Group By A.nEvrakKalemID, A.cEvrakKalem, B.nMiktar
Order By A.nEvrakKalemID

In the above question I want to sum up the Inventory column and group it according to the column nEvrakKalemID. But it does not work. Thank you in advance for your help.

Answer 1

Try this :

Select A.nEvrakKalemID As ID, 
       A.cEvrakKalem As [Inventory Name],
       (SUM(CASE  WHEN B.nEvrakTip = 3 or B.nEvrakTip = 7 
                  THEN (IsNull(B.nMiktar,0))
                  ELSE 0
            END) -
        SUM(CASE WHEN B.nEvrakTip = 4 or B.nEvrakTip = 8
                 THEN IsNull(B.nMiktar,0)
                 ELSE 0
            END)) As Inventory
From MasrafAnaliz.dbo.EvrakKalem A 
Left Outer Join MasrafAnaliz.dbo.EvrakKalemHareket B On B.nKodEvrakKalem = A.nEvrakKalemID
Group By A.nEvrakKalemID, A.cEvrakKalem, B.nMiktar
Order By A.nEvrakKalemID