Aggregates and Joins - Mailing list pgsql-general

Hello,

I've kind of run into a wall, just now, and I don't know if I can climb it
with Postgres.  In order to fully explain my problem, I've got to actually
go into detail about my database setup... so, if you're willing to
listen/read for a minute, I appreciate it.  Just so you know, my problem -
as I see it - is related to the limitation of aggregate function use.

In my database, I have two tables.  In one of the tables, I have a primary
key 'trans_id'.  In the other table, I store vital information associated
with 'trans_id'.  More specifically:

table first_table (

   trans_id int,  -- my primary key
   otherfields blah
   blah blah

);

table second_table (

   this_id int,
   price money,
   ...
   ..
   trans_id int, -- my primary key association

);

In second_table, there may be any number (greater than zero) of records
associated with a single record in first_table (ie. if I have a record in
frist_table with a 'trans_id' = 33, I may have 1 or 100 records in
second_table with a 'trans_id' = 33).

What I need to do is to obtain the sum of the field 'price' from
second_table for every distinct 'trans_id' (all of which can be found in
first_table).  Right now, I do not know of any way to do this, other than
by using my scripting language to first get all of the 'trans_id' values
from first_table - "select trans_id from first_table" - and then (for
each and every one of the records returned) initiating a separate query -
"select sum(price) from second_table where trans_id = onetransid" - that
would use the aggregate function sum() on the price field in second_table.
This isn't a problem if I'm only trying to get the sum(price) for a
hundred or so distinct 'trans_id' values.  However, I will (more often
than not) be trying to do this for thousands of distinct 'trans_id'
values.  Is there a quicker way to do this?  Is it possible to use the
aggregate functions and have more than one row returned?

Here's some pseudo-code to look at, if you'd like:

// set up the query
$query = "select trans_id from first_table";
// execute the query and obtain a pointer to the results
$result = dbExec($postgresConnection, $query);

// find out how many records were returned
$numrecords = pg_NumRows($result);

// obtain the 'trans_id' values, one by one, and sum() the price
for($i = 0; $i < $numrecords; $i++) {

    $trans_id = pg_Result($result, $i, 0);  // record $i, index 0

    $subquery = "select sum(price) from second_table " .
                "where trans_id = $trans_id";
    $subresult = dbExec($postgresConnection, $subquery);

    // only one record - to my knowledge - can be returned
    $pricesum[$trans_id] = pg_Result($result, 0, 0);

}


Any help you can give will be graciously accepted.....

Thanks,

Bob