= List of most popular products with total number of sales
{{{#!sql
CREATE OR REPLACE FUNCTION get_products_by_total_sales()
RETURNS TABLE (
    product_code INT,
    product_description TEXT,
    product_price NUMERIC,
    number_of_orders BIGINT,
    total_revenue NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT
        p.code AS product_code,
        p.description AS product_description,
        p.price AS product_price,
        COUNT(DISTINCT o.order_num) AS number_of_orders,
        COALESCE(
            SUM(
                p.price
                * o.quantity
                * (1 - COALESCE(o.discount, 0) / 100.0)
            ),
            0
        ) AS total_revenue
    FROM product p
    LEFT JOIN includes i
        ON p.code = i.code
    LEFT JOIN "order" o
        ON i.order_num = o.order_num
    GROUP BY
        p.code,
        p.description,
        p.price
    ORDER BY
        number_of_orders DESC,
        total_revenue DESC;
END;
$$;

}}}

== Relational Algebra
- P(code, price, availability, description, ...)
- O(order_num, quantity, status, last_modified_date, payment_method, discount)
- I(code, order_num)

**JOIN products with orders:**
- J1 ← P ⟕P.code = I.code I
- J2 ← J1 ⟕I.order_num = O.order_num O

**Calculate revenue from product for each order:**
- **FORMULA:** order_total = Σ(P.price × O.quantity) × (1 - COALESCE(O.discount, 0) / 100)

- OR ← πcode, description, price, order_num, quantity, discount,
      price × quantity ×
      (1 - COALESCE(discount, 0) / 100)
      → order_revenue(J2)

* OR = order revenue

**Calculate total number of orders the product has been in and total revenue:**
- R ← γcode, description, price;
     COUNT(DISTINCT order_num) → number_of_orders,
     Σ(order_revenue) → total_revenue
     (OR)
- For products that have never been ordered:
    - number_of_orders = 0
    - total_revenue = 0

**Sort by order total:**
- R_final ← τnumber_of_orders DESC, total_revenue DESC(R)