join - SQL not including duplicate values on a combination of 2 columns -

i working on exercise 16 sql-ex.ru. problem asks following:

find pairs of pc models having identical speeds , ram.  result, each resulting pair shown once, i.e. (i, j) not (j, i).  result set: model higher number, model lower number, speed, , ram. 

the database schema :

product(maker, model, type) pc(code, model, speed, ram, hd, cd, price) laptop(code, model, speed, ram, hd, screen, price) printer(code, model, color, type, price) 

i wrote following query:

select a.model, b.model, a.speed, a.ram pc join pc b on (a.model<>b.model) a.speed=b.speed , a.ram=b.ram 

but displays duplicates of i,j j,i. here 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 

as can see, values of i,j flipped , counted distinct values. there easy way rid of duplicates this? kind of lost on part.

i think "model higher number, model lower number" in problem statement clue need have a.model > b.model condition somewhere. join's on condition sounds fine candidate:

select a.model, b.model, a.speed, a.ram pc join pc b on (a.model > b.model) -- <<<=== here a.speed=b.speed , a.ram=b.ram 

the <> symmetrical; > not. switching > ensures if {i, j} in, {j, i} out sure.