Re: Aggregating both tables in a join?

On 1/11/07, Steve Sabljak <sql-ss(at)sabljak(dot)org> wrote:
>
>
>
> select cf.flight_id, tl1.num_legs, cf.cmp_flight_id, tl2.num_legscmp_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 .

--
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Aaron Bono
Aranya Software Technologies, Inc.
http://www.aranya.com
http://codeelixir.com
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