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

Question

I want to create a column element_type in a table (called discussion) that allows the text values \"lesson\" or \"quiz\" but will generate an error if

Answer 1

This trigger throws an exception whenever someone try to insert or update a row with an invalid element_type.

CREATE OR REPLACE FUNCTION check_discussion_element_type() RETURNS TRIGGER AS $$
DECLARE new_element_type varchar(25);
BEGIN
    SELECT element_type into new_element_type
        FROM discussion
        WHERE discussion.element_id = NEW.element_id;

    IF new_element_type != 'lesson' AND new_element_type != 'quiz'
       THEN RAISE EXCEPTION 'Unexpected discussion type';
    END IF;
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;
create trigger t_check_discussion_element_type after update or insert on discussion for each row execute procedure check_discussion_element_type();

If you want to remove the hard-coded types you can adapt it to check if the new type exists in a type table.