Do you absolutely need foreign keys in a database?

Question

I was wondering how useful foreign keys really are in a database. Essentially, if the developers know what keys the different tables depend on, they can write the queries just

Answer 1

Use constraints rather than application logic to enforce integrity because it is generally easier, cheaper and more reliable to maintain constraints in one place (the database) rather than in every application.

I understand from one of your comments that your motivation for asking the question is that you think leaving out the keys may make it easier to evolve the database design during development. In my experience you are wrong about that. I find that it's actually better to be more restrictive with constraints in the early stages of development. If in doubt, create the constraint because it's much easier to remove constraints later than it is to create them. Removing a constraint will tend to break fewer things than adding one and generally requires less testing and fewer code changes to achieve.