Your original method is two queries. You first query for the unique values and then go over every row of it and repeatedly launch a new query to fetch the related value. Fetching related values is usually called a 'join' in relational-database speak.

It happens that relational databases are veeery good at joins. It's what they do for a living. What I did was that I actually changed your two queries into... well, two queries that are linked by a join. That way they will be sent to the database just once, the database figures out the result, and returns it to your program.

The one query in parentheses with the max() is known as a subquery and it returns an intermediate resultset. It's basically a second (temporary) table. It fills the role of your LIMIT 1 query, except that it returns all the related rows rather than just one.

This temporary resultset is then joined against the main table where the id and rpt_key columns match. This gives you the resultset you previously used Perl to build -- in a single query.

You can take the two small queries out of the big query and run them against the database and examine the result to see what they return and how it works.

If you work with relational databases, you really ought to learn SQL to a level that is above the very basic SELECTs and INSERTs. It'll require some stretching of the brain but it's worth it.