= Approximate number of orders per client
{{{#!sql
CREATE OR REPLACE FUNCTION get_approximate_orders_per_client()
RETURNS TABLE (
    total_clients BIGINT,
    total_orders BIGINT,
    approximate_orders_per_client NUMERIC
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT
        (SELECT COUNT(*) FROM client) AS total_clients,
        (SELECT COUNT(*) FROM "order") AS total_orders,
        ROUND(
            (SELECT COUNT(*) FROM "order")::NUMERIC
            / NULLIF((SELECT COUNT(*) FROM client), 0),
            2
        ) AS approximate_orders_per_client;
END;
$$;

}}}

== Relational Algebra
- O(order_num, quantity, status, last_modified_date, payment_method, discount)
- C(client_id, name, first_name, last_name, email, password, delivery_address)

**Calculate total number of orders:**
- O_total ← γCOUNT(order_num) → total_orders(O)

**Calculate total number of clients:**
- C_total ← γCOUNT(client_id) → total_clients(C)

**Calculate approximate number of orders per client:**
- R ← γclient_id, name;
  R ← πtotal_clients,
     total_orders,
     total_orders / total_clients
     → approximate_orders_per_client
     (C_total × O_total)