On 1/11/07,
Steve Sabljak <
sql-ss@sabljak.org> wrote:
select cf.flight_id, tl1.num_legs, cf.cmp_flight_id, tl2.num_legs cmp_num_legs
from cmp_flight cf
join (select fl1.flight_id fid1, count(*) num_legs
from flight_leg fl1 group by fl1.flight_id) tl1 on fid1 = cf.flight_id
join (select fl2.flight_id fid2, count(*) num_legs
from flight_leg fl2 group by fl2.flight_id) tl2 on fid2 =
cf.cmp_flight_id;
Is this a better way to go about it?
I've considered keeping a track of the number of legs in the flight
table, using a trigger, but it doesn't seem relationally 'clean'.
You can try this but I cannot say which would perform better. I have heard many times that "distinct" can be a performance killer.
select
cf.flight_id,
count(distinct(fl1.leg_id)) as num_legs,
cf.cmp_flight_id,
count(distinct(fl2.leg_id)) cmp_num_legs
from cmp_flight cf
join flight_leg fl1 on fl1.flight_id = cf.flight_id
join flight_leg fl2 on fl2.flight_id = cf.cmp_flight_id
group by
cf.flight_id,
cf.cmp_flight_id
;
This can give different results if you don't have a unique constraint (or primary key set to) cmp_flight.flight_id, cmp_flight.cmp_flight_id .
--
==================================================================
Aaron Bono
Aranya Software Technologies, Inc.
http://www.aranya.com http://codeelixir.com==================================================================
