mysql - 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 reside in id i.e. primary key.

I want to retrieve the last distinct set of records in this table based on the datetime value. For example, let's say below is my table

id   code   value   datetime               timestamp
1    1023   23.56   2011-04-05 14:54:52    1234223421
2    1024   23.56   2011-04-05 14:55:52    1234223423
3    1025   23.56   2011-04-05 14:56:52    1234223424
4    1023   23.56   2011-04-05 14:57:52    1234223425
5    1025   23.56   2011-04-05 14:58:52    1234223426
6    1025   23.56   2011-04-05 14:59:52    1234223427
7    1024   23.56   2011-04-05 15:00:12    1234223428
8    1026   23.56   2011-04-05 15:01:14    1234223429
9    1025   23.56   2011-04-05 15:02:22    1234223430

I want to retrieve the records with IDs 4, 7, 8, and 9 i.e. the last set of records with distinct codes (based on datetime value). What I have highlighted is simply an example of what I'm trying to achieve, as this table is going to eventually contain millions of records, and hundreds of individual code values.

What SQL statement can I use to achieve this? I can't seem to get it done with a single SQL statement. My database is MySQL 5.

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.