Nice reminder from Brian Sherwin on the correct way to do grouping without having every field in the Group By  clause.

SELECT C.CustomerID, C.CustomerName, C.City, C.State, S.TotalSales
FROM Customers C
INNER JOIN
(SELECT CustomerID, SUM(Sales) as TotalSales
FROM Sales 
GROUP BY CustomerID) S
ON C.CustomerID = S.CustomerID