Re: Seeking help with a query....

>
> Hi folks, seeking help with a query that I thought was simple, but
> apparantly isn't, at least for someone with my knowledge level.
>
> Given a table :
>
> create table atable (
> code1 char,
> code2 char,
> cost int
> );
>
> And the rows
>
> code1 code2 cost
> -----------------------------
> a b 2
> d e 4
> b a 6
> f g 1
>
> I need a ( preferably single ) query that will sum the costs for any
> matching pairs of codes regardless of order. That is, row 1 and row 3
> concern the same pair of unordered codes (a,b), and the result should
show
> that the (a,b) pair had a summed cost of 8. I am not able to change
any of
> the environment or preconditions other than the query itself. I have
tried
> so many approaches that they aren't even worth listing. Any
suggestions
> would be very much appreciated.
>
My approach is inspired by PostgreSQL 7.3 Set Returning Functions by
Stephan Szabo
( http://techdocs.postgresql.org/guides/SetReturningFunctions )

Use a set returning function to get ordered pairs and do a group by then
as

create type aholder as (
code1 char,
code2 char,
cost int
);

create or replace function aordered() returns setof aholder as '
declare
myrow aholder%ROWTYPE;
codex char;
begin
for myrow in select code1,code2,cost from atable
loop
if myrow.code1 > myrow.code2 then
codex := myrow.code1;
myrow.code1 := myrow.code2;
myrow.code2 := codex;
end if;
return next myrow;
end loop;
return;
end ' language 'plpgsql';

select code1,code2,sum(cost) from aordered() group by code1,code2 ;
code1 | code2 | sum
-------+-------+-----
a | b | 8
d | e | 4
f | g | 1
(3 rows)

In addition if you want only matching pairs (eliminate single pairs) try

create or replace function amatched() returns setof aholder as '
declare
myrow aholder%ROWTYPE;
codex char;
begin
for myrow in select t1.code1,t1.code2,t1.cost from atable t1,atable t2
where t1.code1=t2.code2 and t1.code2=t2.code1
loop
if myrow.code1 > myrow.code2 then
codex := myrow.code1;
myrow.code1 := myrow.code2;
myrow.code2 := codex;
end if;
return next myrow;
end loop;
return;
end ' language 'plpgsql';

select code1,code2,sum(cost) from amatched() group by code1,code2 ;
code1 | code2 | sum
-------+-------+-----
a | b | 8
(1 row)

Hope this helps.
Regards, Christoph