Question

Just a practice problem, little bit confusing I’d appreciate it if somebody could clear this up..

I have a DB called Product and another one called PC. I’m trying to execute a query to “find the average hard disk size of a PC for all those manufacturers who also make printers”

Here’s my code:

SELECT maker, AVG(hd) 
FROM pc, product
WHERE pc.model=product.model
GROUP BY maker
HAVING COUNT(type='PR')>=1

type is an attribute of Product which either stands for printer(PR) laptop or PC, however the HAVING statement doesn’t seem to single out only the makers who produce printers, instead I get back an AVG for each maker in the Product table.

UPDATE

This is what the relations look like:

Product(maker, model, type)
PC(model, hd, speed, ram, price)
Printer(model, color, type, price)

Answer 1

Try the following query:

SELECT maker, AVG(hd) FROM PC, Product
WHERE PC.model=Product.model
AND Product.maker IN 
   (SELECT DISTINCT maker FROM Product WHERE type='PR')
GROUP BY Product.maker;

Demo: http://sqlfiddle.com/#!2/abfaa/2

You simply add a condition to make sure that the maker is one of the makers that have at least one printer product. You then group by the maker to find the individual averages.