expressing one-or-more constraint structurally

When one wants to express a cardinality constraint on one item vs. another, being able to do it structurally is the best approach. The most basic example is one-to-many. If, for each a in A, you have 0 or more b in B, then have B have, in addition to its own primary key, a foreign key to A.

If each a:A has zero-or-one b:B, then have B have an attribute that is both the primary key of B and a foreign key to A.

What if one wants to express a one-or-more constraint? For each a:A there exists at least one b:B? One could always write a check constraint, but that is not as desirable as a structural way to express the same.