John Baptiste of CatsWhoCode.com has published 10 useful SQL queries that you can use to clean up the database that houses your WordPress installation. Before any SQL queries are run, you should definitely back up your entire website as a wrongly executed query can ruin the database, rendering your site useless. If you’re afraid to execute any of the queries mentioned within his list, there are plugins available that can accomplish the same tasks such as WP-Cleanup but the same precautions must be taken, especially since some of the cleaning plugins are old and may not have the correct table or location to delete data from.