Question

I am trying to solve an sql exercise.

Here’s the schema

PC

code     int  
model    varchar(50)  
speed    smallint  
ram      smallint  
hd       real  
cd       varchar(10)  
price    money  

The problem :

Find the pairs of PC models having
similar speeds and RAM. As a result,
each resulting pair is shown only
once, i.e. (i, j) but not (j, i).

I have written a query but it displays (i, j) along with (j, i).

My query :

select  t1.model,t2.model,t1.speed,t1.ram from pc t1 , pc t2
where t1.speed = t2.speed and t1.ram = t2.ram and t1.model != t2.model

Output :

model   model   speed   ram
1121    1233    750     128
1232    1233    500     64
1232    1260    500     32
1233    1121    750     128
1233    1232    500     64
1260    1232    500     32

Required output:

model   model   speed   ram
1233    1121    750     128
1233    1232    500     64
1260    1232    500     32

So how do I avoid (j ,i) in my output?

Thanks.

Answer 1

The difference between your output and the required output are exactly the rows that have t1.model < t2.model. To remove those, just add another AND t1.model >= t2.model. But because you already require that t1.model != t2.model, the complete query is

select  t1.model,t2.model,t1.speed,t1.ram 
from pc t1 , pc t2 
where t1.speed = t2.speed and t1.ram = t2.ram and t1.model > t2.model