CREATE TYPE item_source AS ENUM ('shop', 'neutral');

CREATE TABLE items (
    id SERIAL PRIMARY KEY,
    name TEXT NOT NULL UNIQUE,
    source item_source NOT NULL,
    min_tier SMALLINT,
    max_tier SMALLINT,
    image_url TEXT NOT NULL,
    CONSTRAINT neutral_tier_matches_source CHECK (
        (source = 'neutral' AND min_tier BETWEEN 1 AND 5 AND max_tier BETWEEN 1 AND 5 AND min_tier <= max_tier)
        OR (source = 'shop' AND min_tier IS NULL AND max_tier IS NULL)
    )
);