Chapter 5. Creating XML Schemas

For the past two chapters, we've been working with document type definitions (DTDs). Over time, many people have complained to the World Wide Web Consortium (W3C) about the complexity of DTDs and have asked for something simpler. The W3C listened, assigned a committee to work on the problem, and came up with a solution that is much more complex than DTDs ever were: XML schemas.

On the other hand, schemas are also far more powerful and precise than DTDs ever were. With schemas, you can specify not only the syntax of a document as you would with a DTD, but also the actual data types of each element's content, inherit syntax from other schemas, annotate schemas, use them with multiple namespaces, create simple and complex data types, specify the minimum and maximum number of times an element can occur, create list types, create attribute groups, restrict the ranges of values that elements can hold, restrict what other schemas can inherit from yours, merge fragments of multiple schemas, require that attribute or element values be unique, and much more.