How to delete all rows in a group except the newest one?

Say, I have a table similar to this:

CREATE TABLE `mytable` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `application_id` int(11) NOT NULL,
  `company_name` varchar(100) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB

I want to make application_id unique, but there're some duplicates in the table already. How can I group by application_id and remove all records per group, leaving just the one with the highest id?

You can use a multi-table DELETE with a self-join:

DELETE t1 FROM mytable t1 INNER JOIN mytable t2
WHERE t1.application_id = t2.application_id AND t1.id < t2.id

How it works:

• The table is joined to itself using INNER (CROSS) join; that is, each and every row in the table is joined to each and every row in the same table.
• The WHERE condition filters the rows with the same application_id - this is effectively the "grouping" part - and the ones with lower ids. Those are removed from the target table, keeping only rows with the highest id per application_id.