Listing 1: Creation Script for the fn_concatorders Function
SET NOCOUNT ON;
USE Northwind;
GO

IF OBJECT_ID('dbo.fn_concatorders') IS NOT NULL
  DROP FUNCTION dbo.fn_concatorders;
GO

CREATE FUNCTION dbo.fn_concatorders(@cid AS NCHAR(5))
  RETURNS VARCHAR(MAX)
AS
BEGIN
  DECLARE @ArrOrders AS VARCHAR(MAX);

  SET @ArrOrders = '';

  SELECT @ArrOrders = @ArrOrders + ',' + CAST(OrderID AS VARCHAR(MAX))
  FROM dbo.Orders
  WHERE CustomerID = @cid;
  
  RETURN STUFF(@ArrOrders, 1, 1, '');
END
GO