Unique coverage between a pair of values in SQL - sql

Question

I want to get a distinct list of Item_IDs per vendor pair, e.g. (A,B), (A,C), (B,C)
I have a table like this:
Item_ID | vendor
123 A
123 B
133 B
456 C
I want to list ALL of the distinct Item_IDs between each pair of vendors, like below. Essentially, I want to know all of the Item_IDs which are "covered" (or present) between a pair of vendors.
Vendor1 | Vendor 2 | Item_ID
A B 123
A B 133
A C 123
A C 456
B C 123
B C 133
B C 456
I have tried using a self join, to get 2 vendors side by side, but struggling on how to list out the Item_IDs per pair.
Here is what I have so far:
select
distinct a.vendor as vendor1, b.vendor as vendor2
from table a
join table b on a.vendor != b.vendor
and a.vendor >= b.vendor -- to remove duplicate combinations

Answer 1

The following query works in SQL Server 2019 but I don't have the chance of testing it in Azure.
You can do:
with
v as (select distinct vendor as vendor from t)
select
a.vendor as vendor1,
b.vendor as vendor2,
x.item_id
from v a
join v b on b.vendor > a.vendor
cross apply (
select distinct item_id
from t
where t.vendor = a.vendor or t.vendor = b.vendor
) x
order by a.vendor, b.vendor
Result:
vendor1 vendor2 item_id
-------- -------- -------
A B 123
A B 133
A C 123
A C 456
B C 123
B C 133
B C 456
See running example at db<>fiddle.