How to select similar sets in SQL

Question

I have the following tables:

Order
----
ID (pk)

OrderItem
----
OrderID (fk -> Order.ID)
ItemID (fk -> Item.ID)
Quantity

Item
----
ID (pk)
         


        

Answer 1

The approach that I would take would be to first of all find all orderitems that are 85% similar to the orderitems for the selected order, then count the number of these orderitems for each order and check whether the number of items is 85% similar to the selected order, using the following query:

DECLARE @OrderId int = 2
SET @OrderId = 6

/*
Retrieve orderitems that match 85% with required @orderId
*/
;WITH SelectedOrderItemCount
AS
(
    SELECT COUNT(*) * 0.85 AS LowerBoundary, COUNT(*) * 1.15 AS UpperBoundary
    FROM OrderItem
    WHERE OrderId = @OrderId
)
SELECT OtherOrders.OrderId, COUNT(*) as NumberOfOrderItems
FROM OrderItem SpecificOrder
INNER JOIN OrderItem OtherOrders
    ON OtherOrders.ItemId = SpecificOrder.ItemId
WHERE SpecificOrder.OrderId = @OrderId AND
      OtherOrders.OrderId <> @OrderId AND
    OtherOrders.Quantity BETWEEN SpecificOrder.Quantity * 0.85 AND SpecificOrder.Quantity * 1.15
GROUP BY OtherOrders.OrderId
HAVING COUNT(*) BETWEEN (SELECT LowerBoundary FROM SelectedOrderItemCount) 
                    AND (SELECT UpperBoundary FROM SelectedOrderItemCount)

Full SQLFiddle demo here