Every developer knows the horror of mysql sort/order by or select distinct using with group by. Mysql does group by before order by and you get mixed results not what you expected. This is a small solution with less performance problem:
SELECT * FROM ( select * from `my_table` order by timestamp desc ) as my_table_tmp group by catid order by nid desc
In this example we get latest news in each category. We create a temp table by sorting by timestamp and group by after it. It worked for me.