Question

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

I am trying to find pc’s who share both the same speed and same ram.

What I have tried:

SELECT model FROM pc WHERE pc.speed = pc.speed AND pc.ram = pc.ram;

This is again (If you have seen my list of questions from today), that this is the wrong syntax.

I want to compare each tuple in pc to each other tuple in pc and create a new relation based on a pc’s speed and ram sharing anothers.

Answer 1

SELECT pc1.model, pc2.model 
FROM 
    pc pc1 
    JOIN pc pc2 
        ON pc1.speed = pc2.speed 
        AND pc1.ram = pc2.ram
        AND pc1.model <> pc2.model

Note that this will return two rows for every match… A <-> B and B <-> A. If you want to prevent this, you can change your query to something like the following:

SELECT pc1.model, pc2.model 
FROM 
    pc pc1 
    JOIN pc pc2 
        ON pc1.speed = pc2.speed 
        AND pc1.ram = pc2.ram
        AND pc1.model > pc2.model