a)

SELECT AVG(speed)
FROM PC;

b)

SELECT AVG(speed)
FROM Laptop
WHERE price > 2000;

c)

CREATE TEMPORARY TABLE temp SELECT model
                            FROM Product
                            WHERE maker = "A";

SELECT AVG(price)
FROM temp NATURAL JOIN PC;

d)

CREATE TEMPORARY TABLE temp SELECT model, price FROM PC;
INSERT INTO temp SELECT model, price FROM Laptop;

CREATE TEMPORARY TABLE temp2 SELECT model
                             FROM Product
                             WHERE maker = "D";

SELECT AVG(price)
FROM temp NATURAL JOIN temp2;

e)

SELECT speed, AVG(price)
FROM PC
GROUP BY speed;

f)

SELECT maker, AVG(screen)
FROM Product NATURAL JOIN Laptop
GROUP BY maker;

g)

SELECT maker
FROM Product
WHERE type = "pc"
GROUP BY maker
HAVING COUNT(*) >= 3;

h)

SELECT maker, MAX(price) AS Max_PC_price
FROM Product NATURAL JOIN PC
GROUP BY maker;

i)

SELECT speed, AVG(price) 
FROM PC
WHERE speed > 800
GROUP BY speed;

j)

CREATE TEMPORARY TABLE temp SELECT DISTINCT p1.maker, p1.model AS model
                            FROM Product as p1, Product as p2
                            WHERE p2.type = "printer" AND
                                  p1.type = "pc" AND
                                  p1.maker = p2.maker;

SELECT maker, AVG(hd)
FROM temp NATURAL JOIN PC
GROUP BY maker;