You are not, of course, required to provide values for all columns—other than primary key columns—when inserting a row. If, for instance, we decide to allow users to register without providing an email, we may issue this perfectly valid query:
INSERT INTO "users" ("username", "encrypted_password") VALUES ( 'bob', 0x10920941a69549d33aaee6116ed1f47e19b8e713 );
In the preceding query, we only insert values for the username and encrypted_password fields; the row will have no value in the email field.