=> CREATE OR REPLACE FUNCTION add_author(last_name text, first_name text, middle_name text)
RETURNS integer
AS $$
DECLARE
    author_id integer;
BEGIN
    INSERT INTO authors(last_name, first_name, middle_name)
        VALUES (last_name, first_name, middle_name)
        RETURNING authors.author_id INTO author_id;
    RETURN author_id;
END;
$$ VOLATILE LANGUAGE plpgsql;

CREATE FUNCTION

=> CREATE OR REPLACE FUNCTION buy_book(book_id integer)
RETURNS void
AS $$
BEGIN
    INSERT INTO operations(book_id, qty_change)
        VALUES (book_id, -1);
END;
$$ VOLATILE LANGUAGE plpgsql;

CREATE FUNCTION