I have googled this question, but I do not understand clearly what is an XML schema and DTD (document type definition), and why the XML schema is more powerful compared to DTD. Any guidance would be highly appreciated.
15.9k 34 34 gold badges 118 118 silver badges 211 211 bronze badges asked Oct 9, 2009 at 14:39 103k 119 119 gold badges 272 272 silver badges 367 367 bronze badgesThe critical difference between DTDs and XML Schema is that XML Schema utilize an XML-based syntax, whereas DTDs have a unique syntax held over from SGML DTDs. Although DTDs are often criticized because of this need to learn a new syntax, the syntax itself is quite terse. The opposite is true for XML Schema, which are verbose, but also make use of tags and XML so that authors of XML should find the syntax of XML Schema less intimidating.
The goal of DTDs was to retain a level of compatibility with SGML for applications that might want to convert SGML DTDs into XML DTDs. However, in keeping with one of the goals of XML, "terseness in XML markup is of minimal importance," there is no real concern with keeping the syntax brief.
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So what are some of the other differences which might be especially important when we are converting a DTD? Let's take a look.
Typing
The most significant difference between DTDs and XML Schema is the capability to create and use datatypes in Schema in conjunction with element and attribute declarations. In fact, it's such an important difference that one half of the XML Schema Recommendation is devoted to datatyping and XML Schema. We cover datatypes in detail in Part III of this book, "XML Schema Datatypes."
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Occurrence Constraints
Another area where DTDs and Schema differ significantly is with occurrence constraints. If you recall from our previous examples in Chapter 2, "Schema Structure" (or your own work with DTDs), there are three symbols that you can use to limit the number of occurrences of an element: *, + and ?.
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Enumerations
So, let's say we had a element, and we wanted to be able to define a size attribute for the shirt, which allowed users to choose a size: small, medium, or large. Our DTD would look like this:
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But what if we wanted size to be an element? We can't do that with a DTD. DTDs do not provide for enumerations in an element's text content. However, because of datatypes with Schema, when we declared the enumeration in the preceding example, we actually created a simpleType called size_values which we can now use with an element:
answered Oct 9, 2009 at 15:01 Pascal Thivent Pascal Thivent 569k 140 140 gold badges 1.1k 1.1k silver badges 1.1k 1.1k bronze badgesjust a note, W3C seems to think that DTD is a type of XML schema language: "There are several different schema languages in widespread use, but the main ones are Document Type Definitions (DTDs), Relax-NG, Schematron and W3C XSD (XML Schema Definitions). " w3.org/standards/xml/schema