When i select , only one column is checked without duplicates - oracle

Question

I have a 2 table like this:
first table
+------------+---------------+--------+
| pk | user_one |user_two|
+------------+---------------+--------+
second table
+------------+---------------+--------+----------------+----------------+
| pk | sender |receiver|fk of firsttable|content |
+------------+---------------+--------+----------------+----------------+
First and second table have one to many(1:N) relations.
There are many records in second table:
| pk | sender|receiver|fk of firsttable|content |
|120 |car224 |car223 |1 |test message1 to 223
|121 |car224 |car223 |1 |test message2 to 223
|122 |car224 |car225 |21 |test message1 to 225
|123 |car224 |car225 |21 |test message2 to 225
|124 |car224 |car225 |21 |test message3 to 225
|125 |car224 |car225 |21 |test message4 to 225
I need to find if fk has the same value and I want the row with the largest pk.
I've changed the above column name to make it easier to understand.
Here is the actual sql I've tried so far:
select *
from (select rownum rn,
mr.mrno,
mr.user_one,
mr.user_two,
m.mno,
m.content
from tbl_messagerelation mr,
tbl_message m
where (mr.user_one = 'car224' or
mr.user_two='car224') and
m.rowid in (select max(rowid)
from tbl_message
group by m.mno) and
rownum <= 1*20)
where rn > (1-1) * 20
And this is the result:
+---------+-------+----------+----------+-------------------------+----------------------+
| rn | mrno | user_one | user_two | mno(pk of second table) | content |
+---------+-------+----------+----------+-------------------------+----------------------+
| 1 | 1 | car224 | car223 | 125 | test message4 to 225 |
| 2 | 21 | car224 | car225 | 125 | test message4 to 225 |
+---------+-------+----------+----------+-------------------------+----------------------+
My desired result is something like this:
+---------+---------+----------+--------------------+----------------------+
| fk | sender | receiver | pk of second table | content |
+---------+---------+----------+--------------------+----------------------+
| 1 | car224 | car223 | 121 | test message2 to 223 |
| 21 | car224 | car223 | 125 | test message4 to 225 |
+---------+---------+----------+--------------------+----------------------+

Answer 1

Your table description when compared to your query is confusing me. However, what I could understand was that you are probably looking for row_number().
An important advice is to use standard explicit JOIN syntax rather than outdated a,b syntax for joins. Join keys were not clear to me and you may replace it appropriately in your final query.
select * from
(
select mr.*, m.*, row_number() over ( partition by m.fk order by m.pk desc ) as rn
from tbl_messagerelation mr join tbl_message m on mr.? = m.?
) where rn =1
Or perhaps you don't need that join at all
select * from
(
select m.*, row_number() over ( partition by m.fk order by m.pk desc ) as rn
from tbl_message m
) where rn =1