Question

Relations:

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

I am currently trying to teach myself MySQL for work. I bought a book and am going through some example problems. I have come to a halt trying to figure out how I might structure these queries.

1. Find, for each different speed, the average price of a PC
2. Find, for each manufacturer, the average screen size of its laptops
3. Find the manufacturers that make at least three different models of pc
4. Find, for each manufacturer who sells pc’s, the maximum price of a pc
5. Find, for each speed of pc above 2.0, the average price

I can do the average of a column easily ” SELECT AVG(price) FROM pc ” but I am not sure how to do the for each x find its average of y. Problems 1,2,4,5 are are structured like this, problem 3 I think would have a subquery within it as such, that I think will find a relation with the model, maker of each pc in it. But I am not sure how to do the second part “maker makes at least three different models”

(SELECT model, maker FROM product NATURAL JOIN pc WHERE type = 'pc')

Any help? Thanks in advance!

Answer 1

1/ The for X is done by using a GROUP BY:

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

2/ is quite similar.. can you find it, after having read the GROUP BY Mysql Documentation?

SELECT maker, AVG(speed) 
FROM product p 
JOIN laptop l 
ON p.type="Laptop" AND p.model = l.model
GROUP BY maker

Logic: Same is 1, but here we need the maker. So we need to add the columns from Product. We join product with laptop, and then we group by maker.

3/ is also group by, and you’ll want to use HAVING. Try it, ask in comments if you need more help or a solution.

SELECT maker, COUNT(*)  
FROM Product p 
JOIN PC  
ON p.type="OC" AND p.model = pc.model
GROUP BY maker
HAVING COUNT(*) > 3

Same as 2: join product with PC, now group by maker, now only keep the rows which have a count higher than three. You can indeed do this with a subquery too:

`SELECT maker 
from product p 
where (
  select COUNT(*) 
  from PC 
  where p.type="PC" and p.model = PC.model) 
> 3`

4/ will be a group by, and a join with product to get the maker

Care to try this one again? Your first proposed solution is:

SELECT maker, MAX(DISTINCT price) AS MaxPrice 
FROM (
  SELECT model, maker, price 
  FROM product 
  NATURAL JOIN pc 
  WHERE type = 'pc') AS t5 
GROUP BY maker

• You don’t need a DISTINCT in the MAX, a max is always THE maximum,
  even if two rows have the same highest value. The GROUP BY is a kind
  of distinct, too.
• Your subquery here is not really needed, and if you decide to
  subquery, think about which columns you need (do you need model?
  Why?).
• Care to try the same logic as in 3, with a join?

5/ is like 1, but with a WHERE-clause. No biggie.