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

Hmm, funny, I am currently working on something similar. Why don't you just join the sample order (i.e. their items) with all other orders (their items) and list all orders which have at least 85% matches by grouping the count of matches per order?

-- let @SampleorderID be the ID of a sample

declare @totalOrders int, @ThresholdOrderCount int
select @totalOrders = count(*) from OrderItems where orderID=@SampleOrderID

set @ThresholdOrderCount = 85*@totalOrders/100 -- 85% of the item amount of the sample

-- Now match the contents of the sample order with the contents of all other orders
-- count the #matches and show only those orders with at least 85% identical items
Select AllOrder.OrderID, count(*)
from   OrderItems sample 
       join OrderItems AllOrder 
         on sample.ItemID = AllOrder.ItemID
where sample.OrderID = @SampleOrderID 
  and sample.OrderID<>AllOrder.OrderID
group by AllOrder.OrderID 
having count(*)>@ThresholdOrderCount

This should work. However, it also returns orders, which contain more items than the sample. If that is ok, then the above query should also be quite fast.