order by, but eliminating dupes

So lets say I got a table A(x,y,z, Q) ordered by column Q, with contents
like so:

x y z Q
------------------------
21 5 x 1
21 5 x 2
43 10 t 3
1 2 a 5
43 10 t 10
21 5 x 50

what I would like is to take the results of this query, and maintaining
the order, only list x, y, z , AND
not list any duplicates.

So the result I'd want after getting this query would be:

x y z
----------------
21 5 x
43 10 t
1 2 a

So I'm pulling the results as they come along, and ignore any subsequent
dupes.

I don't know if this is even possible. I've tried combinations of SELECT
DISTINCT, GROUP BY, and others
with
no luck. The best I could do was to do the order by in a subquery, then
do a SELECT UNIQUE on the subquery.
But that kills the order of the subquery. I've tried group by x,y,z but
then I can't ORDER BY Q.

- L