MySQL SELECT statement removes duplicate information for a field _mysql

SELECT statement to remove duplicate information for a field, for example:
Tables Name: Table
ID UID Username Message dateline
1 6 a 111 1284240714 (time stamp)
2 6 a 222 1268840565
3 8 B 444 1266724527
4 9 C 555 1266723391
Execute statement (remove username field repeat information and sort by time):
SELECT *
From Table A INNER JOIN (SELECT Max (dateline) as Dateline
From table GROUP by UID-B on a.dateline = B.dateline
GROUP by ID ORDER by A.dateline DESC
Results:
ID UID Username Message dateline
1 6 a 111 1284240714 (time stamp)
3 8 B 444 1266724527
4 9 C 555 1266723391
Use this statement to display the most recent record information and not allow a message (for example, a user) to appear multiple times (more than once) in one area.
PostScript: Efficiency issues
I started with this statement:
SELECT * FROM table where Dateline in (select Max (Dateline) to table GROUP by UID) Order by Dateline DESC
In: When the volume of data processing is relatively large, there is no efficiency, so the optimization into an inline, calculated faster than 6 times times more ...
Continue to add efficiency on the index ~ ~