Added challenge #4

Challenges/#4 - ufn_GetTotalOrderWeight.sql

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+-- Requirement: ufn_GetTotalOrderWeight
+
+-- Purpose:
+-- We need a scalar function that, given an OrderID, will return the total weight of all the items in that order.
+
+-- Specifics:
+-- The function should be named ufn_GetTotalOrderWeight.
+-- The input to the function should be an OrderID.
+-- The function should return the total weight as a decimal.
+-- The weight of each item can be calculated from the StockItemTransactions table. Each stock item transaction related to an order has a Quantity and is linked to a StockItem which has a TypicalWeightPerUnit.
+-- The total weight for the order can be determined by summing up the products of Quantity and TypicalWeightPerUnit for all stock items in that order.
+
+-- Notes:
+-- Ensure to handle cases where the OrderID might not be present in the database.
+-- Return a value of 0 if there are no items or if the order doesn't exist.
+
+-- Solution: WideWorldImporters\Challenges\Functions\ufn_GetTotalOrderWeight.sql
+
+SELECT TOP 100 OrderID, OrderDate, Challenges.ufn_GetTotalOrderWeight(OrderID) AS TotalOrderWeight FROM Sales.Orders
+ORDER BY OrderDate DESC

WideWorldImporters/Challenges/Functions/ufn_GetTotalOrderWeight.sql

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+CREATE FUNCTION Challenges.ufn_GetTotalOrderWeight ( @OrderID INT )
+RETURNS DECIMAL (18, 3)
+AS
+BEGIN
+    DECLARE @Weight DECIMAL (18, 3)
+    SELECT @Weight = COALESCE(SUM(ol.PickedQuantity * si.TypicalWeightPerUnit),0) FROM Sales.OrderLines AS ol 
+    INNER JOIN Warehouse.StockItems AS si 
+        ON ol.StockItemID = si.StockItemID
+    WHERE ol.OrderID = @OrderID
+    RETURN(@Weight)
+END
+
+GO