As I write this (January 2001), the XML Schema syntax is still in the process of being approved. The W3C is currently considering input from several sources and will likely publish a standard later this year. It's likely that the final XML Schema syntax will differ semantically from XML Data-Reduced (XDR) schemas, the schema syntax currently supported by Microsoft's XML-enabled products (including SQL Server). Microsoft has announced that it will support whatever the final syntax is, so keep an eye out for changes in the technology.
I mentioned earlier that DTDs were somewhat old-fashioned. The reason for this is that there's a newer, better technology for validating XML documents. It's called XML Schema. Unlike DTDs, you build ...