SQL Server: Selecting Value That was met in the Where Criteria - sql-server

Question

My query selects records where given item types were ordered and I would like it to return a column that has the value for the criteria which a given record has met.
For example (since the above explanation is probably confusing):
DECLARE #Item1 VARCHAR(8) = 'Red Shoes',
#Item2 VARCHAR(8) = 'Brown Belt',
#Item3 VARCHAR(8) = 'Blue Shoes',
#Item4 VARCHAR(8) = 'Black Belt'
SELECT DISTINCT ord.Order_number,
ord.Item_number,
ord.Item_type,
ord.Item_desc,
link.Item_number AS linked_item_number
FROM Ordertbl ord
LEFT JOIN Item_tables link
ON link.item_number = ord.item_number
WHERE link.Item_number IN (#Item1,#Item2,#Item3,#Item4) AND
ord.Item_number NOT IN (#Item1,#Item2,#Item3,#Item4)
Desired Outcome: All items that were ordered whenever Item1,2,3, or 4 were ordered and, for each record, a field that depicts what item (1,2,3, or 4) was the source for that record being returned.
Using multiple Union queries with where criteria set to a single item provides the desired outcome if I set the linked_item_number field to the queried item, but that method is less than ideal because, at times, large numbers of items may be queried.

Answer 1

Edited: I've updated my answer a bit, and expanded on a few areas using my best guesses, but hopefully they help help illuminate the points I'm making.
Using NOT IN in a WHERE clause is really bad for performance. It would be better to convert your filters into tables, and then JOIN them to your order table.
But before we get to that, let's make a few DML assumptions here that will help keep things clear. Let's say you have two tables like the following:
CREATE TABLE Ordertbl
(
Order_number INT
,Item_number INT
--you might have more columns in your table
)
CREATE TABLE Item_tables
(
Item_number INT
,Item_type INT
,Item_desc VARCHAR(8)
--again, you might have more columns in your table
)
I'm also going to assume that the details about an item are in Item_tables and not in Ordertbl, because that makes the most sense for a database design to me.
My original answer had the following block of text next:
In this scenario, you'd need two additional tables, one for the list
of Items in where Item_number in (#Item1,#Item2,#Item3,#Item4) which
would have the corresponding Item_numbers. The other table would be
the list of Subjs in Item_number Not in
(#Subj1,#Subj2,#Subj3,#Subj4,#Subj5,#Subj6,#Subj7), again, including
the Item_number for those Subj records.
The question has been updated so that the WHERE clause is different than it was when I wrote the original version of my answer. The design pattern still applies here, even if the variables being used are different.
So let's create a temp table to hold all of our "triggering" items, and then populate it.
DECLARE #Item1 VARCHAR(8) = 'Red Shoes',
#Item2 VARCHAR(8) = 'Brown Belt',
#Item3 VARCHAR(8) = 'Blue Shoes',
#Item4 VARCHAR(8) = 'Black Belt'
CREATE TABLE #TriggeringItems
(
Item_desc VARCHAR(8)
)
INSERT INTO #TriggeringItems
(
Item_desc
)
SELECT #Item1
UNION
SELECT #Item2
UNION
SELECT #Item3
UNION
SELECT #Item4
If you had more filter variables to add, you could keep UNIONing them onto the INSERT.
So now we have our temp table and we can filter our query. Great!
...right?
Well, not quite. Our input parameters are descriptions, and our foreign key is an INT, which means we'll have to do a few extra joins to get our key values into the query. But the general idea is that you'd use an INNER JOIN to replace WHERE ... IN ..., and a LEFT JOIN to replace the WHERE ... NOT IN ... (adding a WHERE X IS NULL clause, where X is the key of the LEFT JOINed table)
If you didn't care about getting the triggering items back in your SELECT, then you could just go ahead with replacing the WHERE ... IN ... with an INNER JOIN, and that would be the end of it. But let's say you only wanted the list of items that WEREN'T the triggering items. For that, you would need to join Ordertbl to itself, to get the list of Order_numbers with triggering items within them. Then you could INNER JOIN one side to the temp table, and LEFT JOIN the other half. I know my explanation might be hard to follow, so let me show you what I mean in code:
SELECT DISTINCT onums.Order_number,
orditems.Item_number,
orditems.Item_type,
orditems.Item_desc,
tinums.Item_number AS linked_item_number
FROM #TriggeringItems ti
INNER JOIN Item_tables tinums ON ti.Item_desc = tinums.Item_desc
INNER JOIN Ordertbl onums ON tinums.Item_number = onums.Item_number
INNER JOIN Ordertbl ord ON onums.Order_number = ord.Order_number
INNER JOIN Item_tables orditems ON ord.Item_number = orditems.Item_number
LEFT JOIN #TriggeringItems excl ON orditems.Item_desc = excl.Item_desc
WHERE excl.Item_desc IS NULL
onums is our list of order numbers, but ord is where we're going to pull our items from. But we only want to return items that aren't triggers, so we LEFT JOIN our temp table at the end and add the WHERE excl.Item_desc IS NULL.