The MySQL SELECT statement removes duplicate information of a field.

Select statement to remove duplicate information of a field, for example:
Table Name: Table
Id uid username message Dateline
1 6 A 111 1284240714 (timestamp)
2 6 A 222 1268840565
3 8 B 444 1266724527
4 9 C 555 1266723391
Execute the statement (remove duplicate username field 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
Result:
Id uid username message Dateline
1 6 A 111 1284240714 (timestamp)
3 8 B 444 1266724527
4 9 C 555 1266723391
This statement is used to display the latest record information. A certain information (such as a user) is not allowed to appear multiple times (more than once) in a region ).
Postscript: efficiency problems
The following statement is used:
Select * from table where Dateline in (select max (Dateline) from Table group by UID) order by Dateline DESC
In: when processing a large amount of data, there is no efficiency at all. Therefore, it is optimized to inline, and the computation speed is more than 6 times faster...
After the query continues, the index is added ~~