source: database/Advanced Reports for database/Each product's monthly sales.txt@ 06ebe74

CREATE OR REPLACE FUNCTION get_products_monthly_sales()
RETURNS TABLE (
    product_code INT,
    product_description TEXT,
    year INT,
    month INT,
    number_of_orders BIGINT,
    total_quantity_sold BIGINT,
    total_revenue NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT
        p.code AS product_code,
        p.description AS product_description,
        EXTRACT(YEAR FROM o.last_modified_date)::INT AS year,
        EXTRACT(MONTH FROM o.last_modified_date)::INT AS month,
        COUNT(DISTINCT o.order_num) AS number_of_orders,
        SUM(o.quantity) AS total_quantity_sold,
        SUM(
            p.price
            * o.quantity
            * (1 - COALESCE(o.discount, 0) / 100.0)
        ) AS total_revenue
    FROM product p
    JOIN includes i
        ON p.code = i.code
    JOIN "order" o
        ON i.order_num = o.order_num
    GROUP BY
        p.code,
        p.description,
        EXTRACT(YEAR FROM o.last_modified_date),
        EXTRACT(MONTH FROM o.last_modified_date)
    ORDER BY
        year DESC,
        month DESC,
        total_revenue DESC;
END;
$$;