SQL needed for getting latest records based on Status?

Question

I have a table TRX which has multiple values TRXID for a given SRCID sample data set shown below.

TRXID       STATUS  TIMESTAMP                           SRCID
839         EN      30-OCT-14 11.08.13.597000000 AM     B0D35D0168G
1189        MO      30-OCT-14 11.13.19.554000000 AM     B0D35D0168G
1549        CA      30-OCT-14 12.13.42.246000000 PM     B0D35D0168G

1666        EN      30-OCT-14 02.43.22.271000000 PM     A0D77E2168G
2221        CA      30-OCT-14 05.49.16.712000000 PM     A0D77E2168G
2244        EN      31-OCT-14 11.21.18.146000000 AM     A0D77E2168G  ...

I want to get all SRCID which have latest status = 'CA' based on latest TIMESTAMP only. so eg if we ran the query for above data set we would only get 'B0D35D0168G' as a result.

Answer 1

This will work in Oracle:

SELECT srcid FROM (
    SELECT srcid, status, ROW_NUMBER() OVER ( PARTITION BY srcid ORDER BY timestamp DESC ) AS rn
      FROM trx
) WHERE status = 'CA' AND rn = 1;

It will work if you need to retrieve additional columns as well (eg, if you need to know what the last value of timestamp is).

SELECT trxid, srcid, timestamp FROM (
    SELECT trxid, srcid, timestamp, status, ROW_NUMBER() OVER ( PARTITION BY srcid ORDER BY timestamp DESC ) AS rn
      FROM trx
) WHERE status = 'CA' AND rn = 1;

Answer 2

Here is the "most recent" pattern I've used quite successfully for several years now. If the key and date field form an index, it is really fast.

select  *
from    trx t1
where   t1.status = 'CA'
   and  t1.timestamp =(
      select  Max( timestamp )
      from    trx
      where   status = t1.status
         and  timestamp <= SysDate );

The last line of the subquery may be omitted if future dates are not possible. Either way, the query should perform only index seeks.