What is difference between XML Schema and DTD ?

Question

Hi,

I have googled this question but I do not understand clearly what is schema and DTD and why Schema is more powerful compared to DTD.

Any guidance would be highly appreciated.

Thanks.

Answer 1

DTD predates XML and is therefore not valid XML itself. That's probably the biggest reason for XSD's invention.

Answer 2

From the Differences Between DTDs and Schema section of the Converting a DTD into a Schema article:

The 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.

[...]

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."

[...]

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 ?.

[...]

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:

<!ELEMENT item (shirt)>
<!ELEMENT shirt (#PCDATA)>
<!ATTLIST shirt
    size_value (small | medium | large)>

[...]

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:

<xs:element name="size" type="size_value">

[...]

Answer 3

DTD is pretty much deprecated because it is limited in its usefulness as a schema language, doesn't support namespace, and does not support data type. In addition, DTD's syntax is quite complicated, making it difficult to understand and maintain..

Answer 4

One difference is also that in a DTD, the content model of an element is completely determined by its name, independently of where it appears in the document. So, say you want to have a name child element of your person element that itself has child elements first and last. Then if you wanted to have a name child element for a city element in the same document, that would also need to have child elements first and last. In contrast, XML Schema allows you to declare child element types locally, so in this case you could declare the name child elements for both person and city separately, giving them their proper content models in those contexts.

The other major difference is support for namespaces. Since DTDs are part of the original XML specification (and inherited from SGML), they are not namespace-aware at all because XML namespaces were specified later. You can use DTDs in combination with namespaces, but it requires some contortions, like being forced to define the prefixes in the DTD and using only those prefixes, instead of being able to use arbitrary prefixes.

To me, other differences are mostly superficial. Datatype support could easily be added to DTDs, and syntax is just syntax. (I, for one, find the XML Schema syntax horrible and would never want to hand-maintain an XML Schema, which I wouldn't say about DTDs or RELAX NG schemas; if I need an XML Schema for some reason, I usually write a RELAX NG one and convert it with trang.)