If said DBMS is tablational, like SQL, then you would have to approximate them using tables and constraints.

If said DBMS is of an another paradigm, like a document database, there may be no way to represent relations within the DBMS.

An enum is a construct that numbers things. There is no way to represent a set of tuples with an integer[1]. I'm not sure where you are trying to go with that one. Inversely, you could hold an enum generated value within a relation. Is that what you mean?

[1] Yes, technically you could break up the individual bits such that they form a set of tuples, but that wouldn't be useful beyond a very narrow use-case and doesn't generalize the way relation implies.