Postgres Constraints

ALTER TABLE public.reservations
ADD CONSTRAINT reservations_room_id_fkey
FOREIGN KEY (room_id) REFERENCES public.rooms(id);

ALTER TABLE public.reservations 
ADD CONSTRAINT reservations_user_id_fkey
FOREIGN KEY (user_id) REFERENCES public.users(id);

ALTER TABLE public.reservations
ADD CONSTRAINT daytime_check check (start_time::time >= '08:00:00'
AND end_time::time <= '17:00:00')

Constraints

Postgres Constraints for Newbies

One of the things that makes Postgres so awesome for software development is the incredibly useful system of constraints. Constraints are a way to tell Postgres which kinds of data can be inserted into tables, columns, or rows. As an application developer, you're going to build in this logic to your application as well and that's great.

https://blog.crunchydata.com/blog/postgres-constraints-for-newbies