SQL
I'm working on a particular tricky SQL query today, so I was looking for some help online. On my travels I bumped into this excellent SQL Tutorial » "This page is a introductory tutorial of the Structured Query Language (also known as SQL) and is a pioneering effort on the World Wide Web, as this is the first comprehensive SQL tutorial available on the Internet. SQL allows users to access data in relational database management systems, such as Oracle, Sybase, Informix, Microsoft SQL Server, Access, and others, by allowing users to describe the data the user wishes to see. SQL also allows users to define the data in a database, and manipulate that data. This page will describe how to use SQL, and give examples." You might also be interested in the SQL part of W3schools and About.com.