Normally I would suggest doing this sort of processing in SQL and then doing what needs to be done on the client side. If you really need to compare every entry with every single other entry that does not have the same code value, you can use (God help me) a Cartesian product:

SELECT a.entry_id, a.some_other_col, b.some_other_col
FROM my_table a, my_table b
WHERE a.entry_code != b.entry_code
ORDER BY a.entry_id
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This way you only need to do one query against the DB. If you have enough memory on the machine, you can also select the data in fairly large chunks using fetchall_arrayref with an appropriate max_rows parameter. But bear in mind what you're trying to do will be inherently fairly resource-consuming. For example, let's say you have 50 groupings and 1600 rows per grouping. That means you'll have a total of (80000 x (80000 - 1600)) = 6,272,000,000 rows of data to deal with, which is a lot.