USE AdventureWorks;
GO
IF OBJECT_ID (N'Sales.ufn_SalesByStore', N'IF') IS NOT NULL
DROP FUNCTION Sales.ufn_SalesByStore;
GO
CREATE FUNCTION Sales.ufn_SalesByStore (@storeid int)
RETURNS TABLE
AS
RETURN 
(
SELECT P.ProductID, P.Name, SUM(SD.LineTotal) AS 'YTD Total'
FROM Production.Product AS P 
JOIN Sales.SalesOrderDetail AS SD ON SD.ProductID = P.ProductID
JOIN Sales.SalesOrderHeader AS SH ON SH.SalesOrderID = SD.SalesOrderID
WHERE SH.CustomerID = @storeid
GROUP BY P.ProductID, P.Name
);
GO

To invoke the function, run this query.

SELECT * FROM Sales.ufn_SalesByStore (602);