XML + database

I just started using XML and I wonder how XML works with a database.

It's acomplicated subject. You can have:

1. A database that stores XML.
2. An intermidiatory language such as PHP changing data from the database output to an XML document for transmission. Then the reverse happening at the other end. This allows for differring databases to work on a standard set of XML data.
3. A database of flat XML files where PHP or something similar does the updates/inserts as SQL would do in a database. There's no reason why PHP couldn't take an SQL statement and apply it to a set of XML documents.

Best thing to do is read the mammoth XML and Databases to find out more.

There are many sources that could help you. IBM DeveloperWorks has a lot of very good tutorilas to get you started - IBM Developerworks XML Tutorials

Here you will find some very detailed information on how to extract data from a database into XML using DOM or SAX. Check these out - they are very good information.