I'm looking to learn about databases and it seems like sql is the way to go. I'm wondering what are the most common sql servers used in businesses today? (ms sql server 2000, mysql ...?) which should i learn? I also here there are different versions of the sql language based on what server you use. which is the most useful version? thanks

MSSQL Server, and Oracle seem to be the predominent engines in most business, although for freelance web development Mysql seems to on the top of the list, possibly because of it's licensing. There is only one version of the sql language, it's a standard. However, Each database engine provides it's own additional features and support some or all of the sql language. Picture it kind of like Learning C, for Lynix, and then trying to write a program for Windows. Most of your code will still be correct, but you will most likely have bits and peices that are Lynix specific and don't work on Windows. Syntax does sometimes change from database to database, but when it does, it's usually easily recognizeable and a quick flip through the online manual will have you back on track. Personally, I found Microsoft SQL Server the easiest to learn, followed closely by MySQL (Which may have been easier, but I was learning PHP at the time along with it). The only other database engines I've used are Interbase, Adaptive Server Anywher, and Microsoft Access. My personal Preference simply from what I use most, is Microsoft SQL Server. I'm a Network Admin/DBA and MSSQL Server works well for me in that position.