Joining one-to-one and one-to-many tables

I'm stumped about the best was to retrieve the most recent entry in a
one-to-many type of table and combine it with a more standard query
that joins one-to-one.

I have defined these:

jross(at)wykids localhost# \d trainer_dates
Table "public.trainer_dates"
Column | Type | Modifiers

--------------------+---------+-----------------------------------------
tr_date_id | integer | not null default
nextval('trainer_dates_tr_date_id_seq'::regclass)
tr_date_short_name | text | not null
tr_date_name | text | not null
tr_date_active | boolean | default false
Indexes:
"trainer_dates_pkey" PRIMARY KEY, btree (tr_date_id)
Referenced by:
TABLE "trainers_trainer_dates" CONSTRAINT
"trainers_trainer_dates_trs_tr_date_id_fkey" FOREIGN KEY (trs_tr_date_id)
REFERENCES trainer_dates(tr_date_id)

and

jross(at)wykids localhost# \d trainers_trainer_dates
Table "public.trainers_trainer_dates"
Column | Type | Modifiers

----------------------+-----------------------------+----------------------
trs_tr_date_pp_id | integer |
trs_tr_date_tr_id | integer |
trs_tr_date_id | integer |
trs_tr_date | date | default ('now'::text)::date
trs_tr_date_recorded | timestamp without time zone | default now()
Foreign-key constraints:
"trainers_trainer_dates_trs_tr_date_id_fkey" FOREIGN KEY (trs_tr_date_id)
REFERENCES trainer_dates(tr_date_id)
"trainers_trainer_dates_trs_tr_date_pp_id_fkey" FOREIGN KEY
(trs_tr_date_pp_id) REFERENCES people(pp_id)
"trainers_trainer_dates_trs_tr_date_tr_id_fkey" FOREIGN KEY
(trs_tr_date_tr_id) REFERENCES trainers(tr_id)

Here are the records in trainer_dates:

jross(at)wykids localhost# select * from trainer_dates;
tr_date_id | tr_date_short_name | tr_date_name | tr_date_active
------------+----------------------------+--------------------+----------------
1 | tr_active_date | Active | t
2 | tr_inactive_date | Inactive | t
3 | tr_destroyed_date | Destroyed | t
4 | tr_pending_date | Pending | t
5 | tr_waiting_for_update_date | Waiting for Update | t
6 | tr_last_updated_date | Last Updated Date | t
7 | tr_application_date | Application Date | t
8 | tr_denied_date | Denied | f
9 | tr_approved_date | Approved | f
(9 rows)

Here is a sample of the data that might be in trainers_trainer_dates:

jross(at)wykids localhost# select trs_tr_date, trs_tr_date_recorded, tr_date_name
from trainers_trainer_dates join trainer_dates on trs_tr_date_id = tr_date_id
where trs_tr_date_tr_id = 1099 order by trs_tr_date_recorded desc;

trs_tr_date | trs_tr_date_recorded | tr_date_name
-------------+----------------------------+-------------------
2010-03-11 | 2010-03-11 09:49:42.736914 | Pending
2009-12-23 | 2009-12-23 01:00:00 | Inactive
2009-12-23 | 2009-12-23 00:00:00 | Last Updated Date
2002-03-21 | 2002-03-21 00:00:00 | Application Date
(4 rows)

(Most of the older trs_tr_date_recorded will not have a full timestamp--this
is a part of a table restructuring and the old table just kept a date. As I
move data into the new tables I add one hour to the timestamp of the current
status to make sure everything sorts correctly.)

The problem comes when I try to offer results to a query that says something
like "show me all trainers whose current status is Pending". This query has
to join three tables, a people table with demographics, a trainers table that
holds a trainer id and some notes and the trainers_trainer_dates table. The
people and trainers table have a one-to-one correlation, while the
trainers_trainer_dates has a one-to-many relationship.

This query, for example, yields 2 results for the trainer referenced above,
instead of just his Pending record:

SELECT
pp_id,
pp_trainer_id,
name,
tr_status,
max(trs_tr_date_recorded)
FROM
(
SELECT
pp_id,
pp_trainer_id,
pp_last_name || ', ' || pp_first_name as name,
trs_tr_date_id,
tr_date_name as tr_status,
trs_tr_date_recorded
FROM people
JOIN trainers_trainer_dates on pp_id = trs_tr_date_pp_id
JOIN trainer_dates on tr_date_id = trs_tr_date_id WHERE
trs_tr_date_id NOT IN (
SELECT tr_date_id from trainer_dates WHERE
tr_date_name in ('Last Updated Date','Application Date')
)
GROUP BY pp_id, pp_trainer_id, pp_last_name, pp_first_name,
trs_tr_date_id, tr_date_name, trs_tr_date_recorded
) as foo2
WHERE pp_id in (
SELECT pp_id from people WHERE
pp_trainer_id IS NOT NULL and pp_provisional_p = 'f'
INTERSECT
SELECT trs_tr_date_pp_id from (
SELECT DISTINCT on (trs_tr_date_pp_id) trs_tr_date_pp_id,
trs_tr_date_id from trainers_trainer_dates
order by trs_tr_date_pp_id asc, trs_tr_date_recorded desc
) as foo
JOIN trainer_dates on tr_date_id = trs_tr_date_id WHERE
tr_date_name = 'Pending'
)
GROUP BY pp_id, pp_trainer_id, name, tr_status
ORDER BY name ASC LIMIT 20 OFFSET 0;

jross(at)wykids localhost# \e
-[ RECORD 1 ]-+---------------------------
pp_id | 2790
pp_trainer_id | 1099
name | Allen, Bryan
tr_status | Pending
max | 2010-03-11 09:49:42.736914
-[ RECORD 2 ]-+---------------------------
pp_id | 2790
pp_trainer_id | 1099
name | Allen, Bryan
tr_status | Inactive
max | 2009-12-23 01:00:00

For my test database, this is the only "Pending" record but in the live
database there will be many trainers with Pending status. I've tried an
initial SELECT DISTINCT ON (pp_id) pp_id but the results of the query are then
piped into a "datatable" that allows sorting on any column so I ran into a
hitch in the gitalong there.

I'm sure there's a better way to accomplish what I'm trying to get here
(especially since this really doesn't quite *work*) but I can't see it. This
type of query has become important for me to get because more and more we are
moving to keeping things in the history type of table so we can keep a
progression of events.

Cluesticks or links welcome!

Thanks!

Jeff Ross