Thread: Aggregating both tables in a join?

Hi,

I've got a table which models the relationship between flights and
legs, which is defined like this:-

create table flight_leg (   flight_id integer references flight(id),   leg_id integer references leg(id)
);

One leg may be part of many flights and one flight may be made up of 1
or more legs.

I also have a table which maps flights to comparable flights. The
mapping is created manually. It is defined like this:-

create table cmp_flight (   flight_id integer references flight(id),   cmp_flight_id integer references flight(id)
);

The flight and leg tables referenced are omitted for brevity.

I want to be able to show all the flight_ids and their leg counts as
well as the cmp_flight_ids and leg counts.

e.g.

insert into flight_legs values (1, 1);
insert into flight_legs values (1, 2);
insert into flight_legs values (2, 3);
insert into flight_legs values (2, 4);
insert into flight_legs values (2, 5);
insert into flight_legs values (3, 3);
insert into flight_legs values (4, 1);
insert into flight_legs values (5, 3);
insert into flight_legs values (5, 4);
insert into flight_legs values (6, 4);
insert into flight_legs values (7, 5);

insert into cmp_flight values (1, 2);
insert into cmp_flight values (1, 3);
insert into cmp_flight values (4, 5);
insert into cmp_flight values (4, 6);
insert into cmp_flight values (4, 7);

The result I'm looking for is

flight_id num_legs cmp_flight_id cmp_num_legs
1         2        2             3
1         2        3             1
4         1        5             2
4         1        6             1
4         1        7             1

This does the trick:-

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
groupby 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'.

Kind Regards,
Steve Sabljak

On 1/11/07, Steve Sabljak <sql-ss@sabljak.org> wrote:
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.
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