Some databases don't even support explicit foreign key constraints, and SQL doesn't make their declaration mandatory. And that metadata is not in the resultset

Some databases don't even support explicit foreign key constraints, and SQL doesn't make their declaration mandatory. And that metadata is not in the resultset, you get an id instead. So SQL dbs will happily let you join with the wrong table. Also foreign key constraints don't play well with sharded SQL dbs. For these reasons, I don't see SQL as a good parallel to HATEOAS. Maybe an ORM would be closer to what HATEOAS brings... It's all about impedance mismatch I think... but HATEOAS is no more a definite solution than ORMs.