#A
SELECT ROUND(AVG(month_price),2) FROM(
  SELECT AVG(price) as month_price, EXTRACT(month FROM data) month FROM purchases p
  JOIN items i on p.itemid = i.itemid WHERE userid IN (
    SELECT userid FROM users WHERE age BETWEEN 18 AND 25
    ) GROUP BY month
   ) as a ;


SELECT ROUND(AVG(month_price),2) FROM(
  SELECT AVG(price) as month_price, EXTRACT(month FROM data) m FROM purchases p
  JOIN items i on p.itemid = i.itemid WHERE userid IN (
    SELECT userid FROM users WHERE age BETWEEN 26 AND 35
    ) GROUP BY m
   ) as a ;

#Б
SELECT month FROM (
  SELECT EXTRACT(month FROM date) month, SUM(items.price) price_month FROM purchases p
  JOIN items i ON p.itemid = i.itemid
  JOIN users u ON p.userid = u.userid
  WHERE u.age >= 35 GROUP BY month
  ) AS a ORDER BY a.price_month DESC LIMIT 1 ;

#В

SELECT top_item FROM (
  SELECT i.itemid top_item, SUM(i.price) f_price FROM items i
  JOIN purchases p ON p.itemid = i.itemid
  WHERE date > ( now()- interval '1 year') GROUP BY top_item
  ) ORDER BY f_price DESC LIMIT 1 ;

#Г
SELECT a.top, CONCAT(a.f_price*100/(SELECT SUM(price) FROM items), '%') AS percent FROM (
  SELECT i.itemid AS top, SUM(price) AS f_price FROM items i
  JOIN purchases p ON i.itemid = p.itemid
  GROUP BY top ORDER BY f_price DESC
  ) AS a LIMIT 3;