Your understanding is correct.
You would do this in many cases. One example is in a relationship like OrderHeader
and OrderDetail
. The PK in OrderHeader
might be OrderNumber
. The PK in OrderDetail
might be OrderNumber
AND LineNumber
. If it was either of those two, it would not be unique, but the combination of the two is guaranteed unique.
The alternative is to use a generated (non-intelligent) primary key, for example in this case OrderDetailId
. But then you would not always see the relationship as easily. Some folks prefer one way; some prefer the other way.