How to run SQL code on a subsets of data (Grouped By without aggregation)

Question

I'm doing some data conversion and I'm required to group my data and perform some logical code on each group. For the example below, I want to group by customer name and customer ID and anytime this group contains the source system as SAP, i want to survive that row and not the others. I've been playing around with row_number() over partition by to group my data but I'm finding it difficult to do what I need to do unless I can run some logic on a group basis. Any help would be greatly appreciated!

Source System    Customer_ID  Customer_Name Survivor 
------------------------------------------------------ 
SAP              1               Walmart         Y  
Oracle           1               Walmart         N  
Oracle           1               Walmart         N  
Oracle           1               Walmart         N  
Oracle           2               Target          Y  
Oracle           2               Target          N  
Oracle           2               Target          N

Answer 1

The following matches your expected output for survivor, but I am not sure of your logic for customer 2 as all rows seem the same:

SELECT  "Source System",
        "Customer ID",
        "Customer Name",
        CASE WHEN ROW_NUMBER() OVER(PARTITION BY "Customer ID" 
                                    ORDER BY CASE WHEN "Source System" = 'SAP' 
                                                THEN 0 
                                                ELSE 1 
                                            END)
                             = 1 THEN 'Y' ELSE 'N' END AS "Survivor"
FROM    T

Example on SQL Fiddle