Customer Count Query

Question

Hello All
I need help in writing a Optimized query .
Scenario :
We have several customers across multiple vendors .We want to get Customercount by vendor for a year 
In this scenario we wanted to claculate 
Customer Count for Vendor 
Example 
2014  Kroger(Vendor) has 100 customers
2014  Target(Vendor) has 50 customers
I wanted to see out of those 100 customers for Kroger who were not customer of kroger in 2012 and 2013 but customer of target
Similarly  out of those 50 customers for Target who were not customer of target in 2012 and 2013 but customer of
kroger
In 2012 same customer might have bought with different vendor.We are calculating new to vendor 
C1  2012   Kroger 100
C1  2012   Walmart 100
C2  2012   target 100
C2  2012   Kroger 100
C1  2014   target 100
C2  2014   Kroger 100
C2  2014   target 100
C2  2014   walmart 100
In this case C1 is customer count for target   as he did not buy with target but with other vendors
and c2 canot be counted towarsd kroger or walmart becauase be bought with both of them but can be counterd towarrds walmart as he did not buy with walmart
I am trying to write a query to get 2012 and 2013 data for all vendors and then join with 2014 data set
but the thing is i am getting wrong count 
Please let me know

Answer 1

this?
SELECT Vendor,
YEAR(DATEADD(yy,DATEDIFF(yy,0,Datefield),0)) AS Yr,
COUNT(CustomerID) AS CustomerCount,
COUNT(CASE WHEN Cnt = 0 THEN CustomerID ELSE NULL END) AS FirstTimeCustomerCount
FROM Table t
OUTER APPLY (SELECT COUNT(*) AS Cnt
FROM Table
WHERE CustomerID = t.CustomerID
AND DATEDIFF(yy,0,Datefield) < DATEDIFF(yy,0,t.Datefield)
AND Vendor = t.Vendor)
GROUP BY Vendor,DATEDIFF(yy,0,Datefield)
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