Get last distinct set of records

This should work for you.

 SELECT * 
 FROM [tableName] 
 WHERE id IN (SELECT MAX(id) FROM [tableName] GROUP BY code)

If id is AUTO_INCREMENT, there's no need to worry about the datetime which is far more expensive to compute, as the most recent datetime will also have the highest id.

Update: From a performance standpoint, make sure the id and code columns are indexed when dealing with a large number of records. If id is the primary key, this is built in, but you may need to add a non-clustered index covering code and id.

I'll try something like this :

select * from table
where id in (
    select id
    from table
    group by code
    having datetime = max(datetime)
)

(disclaimer: this is not tested)

If the row with the bigger datetime also have the bigger id, the solution proposed by smdrager is quicker.

It's and old post, but testing @smdrager answer with large tables was very slow. My fix to this was using "inner join" instead of "where in".

SELECT * 
 FROM [tableName] as t1
 INNER JOIN (SELECT MAX(id) as id FROM [tableName] GROUP BY code) as t2
 ON t1.id = t2.id

This worked really fast.

Try this:

SELECT * 
  FROM <YOUR_TABLE>
 WHERE (code, datetime, timestamp) IN
 (
   SELECT code, MAX(datetime), MAX(timestamp)
     FROM <YOUR_TABLE>
    GROUP BY code
 )