PostgreSQL Guide

Database Design

How can I restrict the possible values for a column?

You have two options:

1. A CHECK CONSTRAINT:

ALTER TABLE distributors
   ADD CONSTRAINT check_types
   CHECK (element_type = 'lesson' OR element_type = 'quiz');

3. A ENUM:

CREATE TYPE element_type AS ENUM ('lesson', 'quiz');

See:
In Postgres, how do you restrict possible values for a particular column?