source: database/Advanced Database Developement/Triggers/Prevention of deleting stores with existing orders or reports.txt@ 6c7cfa6

CREATE OR REPLACE FUNCTION prevent_store_deletion()
RETURNS TRIGGER
LANGUAGE plpgsql
AS $$
BEGIN
    -- Check whether the store has existing reports
    IF EXISTS ( SELECT 1 FROM report WHERE store_id = OLD.store_id ) THEN
        RAISE EXCEPTION 'Store % cannot be deleted because it has existing reports.', OLD.store_id;
    END IF;
    -- Check whether the store has products associated with it
    IF EXISTS ( SELECT 1 FROM sells WHERE store_id = OLD.store_id ) THEN
        RAISE EXCEPTION 'Store % cannot be deleted because it has existing product records.', OLD.store_id;
    END IF;
    RETURN OLD;
END;
$$;

CREATE TRIGGER trg_prevent_store_deletion
BEFORE DELETE
ON store
FOR EACH ROW
EXECUTE FUNCTION prevent_store_deletion();