Question

I know I can do this with a CTE or another means but I am wondering if this can be done in one select query (no sub-selects). I want to find the most recent crt_ts for the po_nbr and then take the associated id.

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Note:

The id column is not guaranteed to be sequential, I simplified it for this question.

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Code:

create table temp
(
    id int,
    po_nbr int,
    crt_ts datetime
)

insert into temp values (20, 100, '09/01/2009')
insert into temp values (3, 100, '09/03/2009')
insert into temp values (5, 100, '09/05/2009')
insert into temp values (6, 100, '09/07/2009')
insert into temp values (4, 200, '08/01/2009')
insert into temp values (29, 200, '08/03/2009')
insert into temp values (12, 200, '08/05/2009')
insert into temp values (18, 200, '08/07/2009')

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Desired result:

id  po_nbr
---------
6   100
18  200

Answer 1

SELECT
  MAX(id) id,
  po_nbr
FROM
  temp
GROUP BY
  po_nbr

To have the associated date, you could do (beware, this implies a sequential id):

SELECT
  temp.id,
  temp.po_nbr,
  temp.crt_ts
FROM
  temp
  INNER JOIN (
    SELECT MAX(id) id FROM temp GROUP BY po_nbr
  ) latest ON latest.id = temp.id

Without a sequential id, it would be:

SELECT
  MAX(temp.id) id,
  temp.po_nbr,
  temp.crt_ts
FROM
  temp INNER JOIN (
    SELECT   MAX(crt_ts) crt_ts, po_nbr 
    FROM     temp i
    GROUP BY po_nbr
  ) latest ON latest.crt_ts = temp.crt_ts AND latest.po_nbr = temp.po_nbr
GROUP BY
  temp.po_nbr,
  temp.crt_ts

The GROUP BY can be left out if there are guaranteed to be no two equal dates per po_nbr group.

Indexes on crt_ts and po_nbr help in the last query, creating one combined index would be best.