Select row that has max total value SQL Server

Question

I have the following scheme (2 tables):

Customer (Id, Name) and Sale (Id, CustomerId, Date, Sum)

How to select the following data ?

1) Best customer of all time (Customer, which has Max Total value in the Sum column)

For example, I have 2 tables (Customers and Sales respectively):

id    CustomerName      
---|--------------
1  | First         
2  | Second 
3  | Third 



id  CustomerId  datetime     Sum
---|----------|------------|-----
1  | 1        | 04/06/2013 | 50
2  | 2        | 04/06/2013 | 60
3  | 3        | 04/07/2013 | 30
4  | 1        | 03/07/2013 | 50
5  | 1        | 03/08/2013 | 50
6  | 2        | 03/08/2013 | 30
7  | 3        | 24/09/2013 | 20

Desired result:

CustomerName TotalSum
------------|--------
 First      | 150

2) Best customer of each month in the current year (the same as previous but for each month in the current year)

Thanks.

Answer 1

Try this for the best customer of all times

SELECT Top 1 WITH TIES c.CustomerName, SUM(s.SUM) AS TotalSum
FROM Customer c JOIN Sales s ON s.CustomerId = c.CustomerId
GROUP BY c.CustomerId, c.CustomerName
ORDER BY SUM(s.SUM) DESC

Answer 2

One option is to use RANK() combined with the SUM aggregate. This will get you the overall values.

select customername,  sumtotal
from (
  select c.customername, 
    sum(s.sum) sumtotal,
    rank() over (order by sum(s.sum) desc) rnk
  from customer c
    join sales s on c.id = s.customerid
  group by c.id, c.customername
  ) t
where rnk = 1

• SQL Fiddle Demo

Grouping this by month and year should be trivial at that point.