a)

SELECT maker, speed
FROM Product NATURAL JOIN Laptop
WHERE hd >= 30;

or

SELECT maker, speed
FROM Product, Laptop
WHERE Product.model = Laptop.model AND hd >= 30;

b)

In standard SQL, one possibility is:

SELECT model, price
FROM (((SELECT model, price FROM PC) UNION
      (SELECT model, price FROM Laptop) UNION
      (SELECT model, price FROM Printer))
      NATURAL JOIN
      Product)
WHERE maker = "B";

In mySQL, set operations are not implemented in version 3.23 installed
at Daimi (but are in the recent version 4.0). Also, sub-selections are
not part of mySQL yet (they are being implemented in the forthcoming
version 4.1). One can instead use temporary relations (relations which
are removed after the session is over):

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

SELECT temp.model, price
FROM temp NATURAL JOIN Product
WHERE maker = "B";

If you want to use temp for other similar things, clean it up:

DROP TABLE temp;

Or better, do this as the first line (to ensure empty relation at
start). If this is your standard policy, you don't need to clean up
after use.

c)

In mySQL:

CREATE TEMPORARY TABLE PCMakers
  SELECT maker FROM Product WHERE type = "pc";

CREATE TEMPORARY TABLE LaptopMakers
  SELECT maker FROM Product WHERE type = "laptop";

SELECT DISTINCT LaptopMakers.maker
FROM LaptopMakers NATURAL LEFT OUTER JOIN PCMakers
WHERE PCMakers.maker IS NULL;

The LEFT OUTER JOIN trick is needed because mySQL does not have the
boolean function [NOT] IN. The trick is described in section 1.7.4.1
in the mySQL documentation (http://www.mysql.com/doc/en/index.html).

d)

SELECT DISTINCT pc1.hd
FROM PC AS pc1, PC AS pc2
WHERE (pc1.hd = pc2.hd) AND (pc1.model != pc2.model);

or (probably better)

SELECT hd
FROM PC
GROUP BY hd
HAVING COUNT(*) >= 2;

e)

SELECT pc1.model, pc2.model
FROM PC AS pc1, PC AS pc2
WHERE (pc1.model < pc2.model) AND 
      (pc1.speed = pc2.speed) AND
      (pc1.ram = pc2.ram);

f)

In mySQL:

CREATE TEMPORARY TABLE temp SELECT model FROM PC WHERE speed >= 1000;
INSERT INTO temp SELECT model FROM Laptop WHERE speed >= 1000;

SELECT maker
FROM Product NATURAL JOIN temp
GROUP BY maker
HAVING COUNT(*) >= 2;

As in the first version of d), we could do the GROUP BY...HAVING part
by using joins and conditions in WHERE (if we want to use only SQL
described up to Section 6.2).

In standard SQL, we could of course pack the entire thing into one big
query (with subqueries).