SQL Server - identify combinations of values and assign combination identifier - sql-server

Question

I am trying to assign what amounts to a 'combinationid' to rows of my table, based on the values in the two columns below. Each product has a number of customers linked to it. For every combination of customers, I need to create a combination ID.
For example, the combination of customers for product 'a' is the same combination of customers for product 'c' (they both have customers 1, 2 and 3), so products a and c should have the same combination identifier ('customergroup'). However, products should not share the same customergroup if they only share some of the same customers - e.g. product b only has customers 1 and 2 (not 3), so should have a different customergroup to products 'a' and 'c'.
Input:
| productid | customerid |
|-----------|------------|
| a | 1 |
| a | 2 |
| a | 3 |
| b | 1 |
| b | 2 |
| c | 3 |
| c | 2 |
| c | 1 |
| d | 1 |
| d | 3 |
| e | 1 |
| e | 2 |
| f | 1 |
| g | 2 |
| h | 3 |
Desired output:
| productid | customerid | customergroup |
|-----------|------------|---------------|
| a | 1 | 1 |
| a | 2 | 1 |
| a | 3 | 1 |
| b | 1 | 2 |
| b | 2 | 2 |
| c | 3 | 1 |
| c | 2 | 1 |
| c | 1 | 1 |
| d | 1 | 3 |
| d | 3 | 3 |
| e | 1 | 2 |
| e | 2 | 2 |
| f | 1 | 4 |
| g | 2 | 5 |
| h | 3 | 6 |
or just
| productid | customergroupid |
|-----------|-----------------|
| a | 1 |
| b | 2 |
| c | 1 |
| d | 3 |
| e | 2 |
| f | 4 |
| g | 5 |
| h | 6 |
Edit: first version of this did include a description of my attempts. I currently have nested queries that basically give me a column for customer 1, 2, 3 etc and then uses dense rank to get the grouping. The problem is that is not dynamic for different numbers of customers and I did not know where to start for getting a dynamic result as above. Thanks for the replies.

Answer 1

Considering you haven't shown your efforts, or confirmed the version you're using, I've assumed you have the latest ("and greatest") version of SQL Server, which means you have access to STRING_AGG.
This doesn't give the groupings in the same order, but I'm going to also also that doesn't matter, and the grouping is just arbitrary. This gives you the following:
WITH VTE AS(
SELECT *
FROM (VALUES('a',1),
('a',2),
('a',3),
('b',1),
('b',2),
('c',3),
('c',2),
('c',1),
('d',1),
('d',3),
('e',1),
('e',2),
('f',1),
('g',2),
('h',3)) V(productid,customerid)),
Groups AS(
SELECT productid,
STRING_AGG(customerid,',') WITHIN GROUP (ORDER BY customerid) AS CustomerIDs
FROM VTE
GROUP BY productid),
Rankings AS(
SELECT productid,
CustomerIDs,
DENSE_RANK() OVER (ORDER BY CustomerIDs ASC) AS Grouping
FROM Groups)
SELECT V.productid,
V.customerid,
R.Grouping AS customergroupid
FROM VTE V
JOIN Rankings R ON V.productid = R.productid
ORDER BY V.productid,
V.customerid;
db<>fiddle.
If you aren't using SQL Server 2017, I suggest looking up the FOR XML PATH method for string aggregation.

Answer 2

Using Larnu's answer this is how I got the result for 2008:
WITH VTE AS(
SELECT *
FROM (VALUES('a','1'),
('a','2'),
('a','3'),
('b','1'),
('b','2'),
('c','3'),
('c','2'),
('c','1'),
('d','1'),
('d','3'),
('e','1'),
('e','2'),
('f','1'),
('g','2'),
('h','3')) V(productid,customerid)),
Groups AS(
SELECT productid, CustomerIDs = STUFF((SELECT N', ' + customerid
FROM VTE AS p2
WHERE p2.productid = p.productid
ORDER BY customerid
FOR XML PATH(N'')), 1, 2, N'')
FROM VTE AS p
GROUP BY productid),
Rankings AS(
SELECT productid,
CustomerIDs,
DENSE_RANK() OVER (ORDER BY CustomerIDs ASC) AS Grouping
FROM Groups)
SELECT V.productid,
V.customerid,
R.Grouping AS customergroupid
FROM VTE V
JOIN Rankings R ON V.productid = R.productid
ORDER BY V.productid,
V.customerid;
Thanks again for your assistance.