How to add a query if the condition in the case is true?

Question

SQL Server

(SELECT c.ClassifierGroupId FROM ClassifierGroup c WHERE c.ClassifierId=
    (CASE WHEN @SupplierContractTypeClassifierGroupId = 0 THEN
      (2)
      ELSE
      (159)
      END)
    )

how do i write

AND c.ClassifierGroupId = @SupplierContractTypeClassifierGroupId

if c.ClassifierId = 2

Answer 1

SELECT ClassifierGroupId 
  FROM ClassifierGroup 
  WHERE 
    (CASE WHEN @SupplierContractTypeClassifierGroupId = 0 THEN
      IIF((ClassifierId=2) AND (ClassifierGroupId = @SupplierContractTypeClassifierGroupId), 1, 0)
      ELSE
      IIF(ClassifierId=159, 1, 0)
      END) = 1