Mysql metadata
If you’re a developer and use mysql, I’m sure you’re aware that it’s a database and it quite good at storing data, but one of the neat things about Mysql (and most other databases) is also their ability to provide meta-data on the contents of the database.
Most people know how to use the meta-data queries in the commandline, but if you want you can also use them in your (php/perl/some-other- ) language. Here is a quick guide to some of them.