Recently, on del.icio.us mailinglist, I asked the question “Does anyone know the database schema of del.icio.us?” .
I got a few private responses so I wanted to share the knowledge with the world.

The Problem: You want to have a database schema where you can tag a bookmark (or a blog post or whatever) with as many tags as you want. Later then, you want to run queries to constrain the bookmarks to a union or intersection of tags. You also want to exclude (say: minus) some tags from the search result.