Get last distinct set of records

Question

I have a database table containing the following columns:

id   code   value   datetime   timestamp

In this table the only unique values res

Answer 1

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.