How to remove duplicate rows from a join query in oracle?

Question

I have two tables bc_transactions & bc_messages. I used the below query to join these two tables

SELECT distinct(bt.USER_TRANS_ID),bm.TS,bm.STATUS
FROM bc_transactions bt
inner JOIN bc_messages bm
ON bt.USER_TRANS_ID=bm.USER_MESSAGE_ID
where bt.protocol_name  = 'Gateway'
and bt.STATUS=bm.STATUS
AND bt.startdate >=TRUNC(SYSDATE-2)
AND bt.startdate <=TRUNC(SYSDATE-1)
AND bt.STATUS like 'ERROR TRANSPORT'
AND bt.HOSTNAME='HEB'
order by bt.USER_TRANS_ID ASC;

the bc_messages table has multiple rows with status as ERROR TRANSPORT which have the same USER_MESSAGE_ID at different timestamp(bm.TS). I am trying to get only the latest row.

|USER_TRANS_ID |            TS        |   STATUS            |
-------------------------------------------------------------
| ID1          | 10-03-2020  15:01:23 |   ERROR TRANSPORT   |
| ID1          | 10-03-2020  15:15:23 |   ERROR TRANSPORT   |
| ID1          | 10-03-2020  15:30:23 |   ERROR TRANSPORT   |
| ID1          | 10-03-2020  15:35:23 |   ERROR TRANSPORT   |
| ID2          | 10-03-2020  16:10:23 |   ERROR TRANSPORT   |
| ID2          | 10-03-2020  16:11:23 |   ERROR TRANSPORT   |

Answer 1

You can use row_number() :

select t.*
from (select bt.USER_TRANS_ID, bm.TS,bm.STATUS,
             row_number() over (partition by bt.USER_TRANS_ID order by bm.TS desc) as seq
      from bc_transactions bt inner join
            bc_messages bm
            on bt.USER_TRANS_ID = bm.USER_MESSAGE_ID
       where bt.protocol_name  = 'Gateway' and 
             bt.STATUS=bm.STATUS and 
             bt.startdate >= TRUNC(SYSDATE-2) and 
             bt.startdate <= TRUNC(SYSDATE-1) and 
             bt.STATUS like 'ERROR TRANSPORT' and 
             bt.HOSTNAME = 'HEB'
     ) t
where seq = 1;