/* Задание 1 */

SELECT 
	category,
	count(*) AS products_count 
FROM 
	products p
GROUP BY category 
ORDER BY products_count DESC;


/* Задание 2
 * Тут всё решение в задании 
 *  */

SELECT 
	order_id,
	sum(quantity) AS total_quantity,
	sum(quantity * price_at_purchase) AS order_revenue
FROM 
	order_items oi 
GROUP BY order_id 
ORDER BY order_revenue DESC;


/* Задание 3 */

SELECT 
	p.category,
	COALESCE(sum(oi.quantity * oi.price_at_purchase), 0) AS category_revenue
FROM
	products p
LEFT JOIN
	order_items oi ON p.id = oi.product_id 
GROUP BY p.category 


/* Задание 4 */

SELECT 
	o.id AS order_id,
	sum(oi.quantity * oi.price_at_purchase) AS order_revenue 
FROM
	orders o 
LEFT JOIN
	order_items oi ON o.id = oi.order_id  
WHERE o.status = 'paid'
GROUP BY 1
ORDER BY 2 DESC 


/* Задание 5 */

SELECT 
	p.category,
	COALESCE(sum(oi.quantity * oi.price_at_purchase), 0) AS category_revenue
FROM
	products p
LEFT JOIN
	order_items oi ON p.id = oi.product_id 
GROUP BY 
	p.category 
HAVING  
	sum(oi.quantity * oi.price_at_purchase) > 1;


/* Задание 6 */

SELECT 
	country,
	count(*) AS active_users_count 
FROM 
	users u 
WHERE
	is_active = TRUE 
GROUP BY 
	country
HAVING 
	count(is_active) > 2;


/* Задание 7 */

SELECT 
	p.name AS product_name,
	COALESCE (sum(oi.quantity), 0) AS total_quantity_sold 
FROM
	products p 
LEFT JOIN
	order_items oi ON p.id = oi.product_id 
GROUP BY p.name
ORDER BY 2 DESC 
LIMIT 3


/* Задание 8 
 * Условие для HAVING в задаче кривое, < 100 ничего нет
 * */

SELECT 
	order_id,
	SUM(quantity * price_at_purchase) AS order_revenue 
FROM	
	order_items oi 
GROUP BY
	order_id
HAVING 
	SUM(quantity * price_at_purchase) < 10000
ORDER BY 2;