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

Solution 1:

You could add a CHECK CONSTRAINT:

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

Although IMO the cleaner option would be to create an ENUM:

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

Solution 2:

A shorcut syntax is :

ALTER TABLE distributors  
   ADD CONSTRAINT check_types 
   CHECK (element_type IN ('lesson', 'quiz') );

This translates automaticolly to :

CONSTRAINT check_types CHECK (element_type::text = ANY (ARRAY['lesson'::character varying, 'quiz'::character varying) )

Enjoy ;-)

Solution 3:

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.