Custome Functions In MySQL Exercise

SQL
1 
-- In sql_store database, create function get_percent_shipped_orders_for_customer(). 
-- No exercises for creating functions are given in the course, so I thought
-- of this challenge in order to practice. 
-- Don't forget to include the appropriate attributes,
-- such as DETERMINISTIC, READS SQL DATA, MODIFIES SQL DATA. 

USE sql_store;
DROP FUNCTION IF EXISTS get_percent_shipped_orders_for_customer;
DELIMITER $$
CREATE FUNCTION get_percent_shipped_orders_for_customer
(
	customer_id INT
)
RETURNS DECIMAL(9,2)
READS SQL DATA
BEGIN
	DECLARE count_of_all_orders INT;
    DECLARE count_of_shipped_orders INT;
    DECLARE percent_shipped_orders DECIMAL(9,2);
    
    SELECT COUNT(*) 
    INTO count_of_all_orders
    FROM orders o
    WHERE
		o.customer_id = customer_id;

	SELECT COUNT(*)
    INTO count_of_shipped_orders
    FROM orders o
    WHERE
        o.customer_id = customer_id AND
		shipped_date IS NOT NULL;
	
    IF count_of_all_orders = 0 THEN
		RETURN 0;
	ELSE
		RETURN ROUND(count_of_shipped_orders/count_of_all_orders, 2); 
	END IF;
END$$
DELIMITER ;

I then used this custom function to create a Percent Shipped Orders for all customers:

USE sql_store;
SELECT 
	CONCAT_WS(" ", c.first_name, c.last_name) AS customer,
    get_percent_shipped_orders_for_customer(c.customer_id) AS pct_shipped_orders
FROM 
	customers c
ORDER BY pct_shipped_orders DESC, customer;

I obtain the following report:
percent_shipped_orders report