Find rows where one column has multiple matching values in another column

Experts,

I have a products table in our MS SQL Server db.

ID, productID, productname, catalogID

The business rule we are going to start using says that each catalogID can have one and only one matching productID.

One productID can have the same catalogID, but not the other way around.

Eventually we will create separate tables with foreign keys, but for now all the columns are in the same table.

How would I find those catalogIds that have more than one matching productId?

For some reason I can't get my head around this.

Thanks in advance.
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bobinorlandoAsked:
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Scott PletcherSenior DBACommented:
I think this will do show you the catalogs with multiple products.

SELECT tn.catalogId, COUNT(DISTINCT tn.productID) AS product_count
FROM tablename tn
GROUP BY tn.catalogid
HAVING COUNT(DISTINCT tn.productID) > 1

To see the details, you can do this:
SELECT *
FROM tablename tn2
WHERE catalogId in (
    SELECT tn.catalogId
    FROM tablename tn
    GROUP BY tn.catalogid
    HAVING COUNT(DISTINCT tn.productID) > 1
    )

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bobinorlandoAuthor Commented:
That did the trick! Thank you!!
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