The answer is
Differences between an XML Schema Definition (XSD) and Document Type Definition (DTD) include:
- XML schemas are written in XML while DTD are derived from SGML syntax.
- XML schemas define datatypes for elements and attributes while DTD doesn't support datatypes.
- XML schemas allow support for namespaces while DTD does not.
- XML schemas define number and order of child elements, while DTD does not.
- XML schemas can be manipulated on your own with XML DOM but it is not possible in case of DTD.
- using XML schema user need not to learn a new language but working with DTD is difficult for a user.
- XML schema provides secure data communication i.e sender can describe the data in a way that receiver will understand, but in case of DTD data can be misunderstood by the receiver.
- XML schemas are extensible while DTD is not extensible.
Not all these bullet points are 100% accurate, but you get the gist.
On the other hand:
- DTD lets you define new ENTITY values for use in your XML file.
- DTD lets you extend it local to an individual XML file.
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..
Similarities:
DTDs and Schemas both perform the same basic functions:
- First, they both declare a laundry list of elements and attributes.
- Second, both describe how those elements are grouped, nested or used within the XML. In other words, they declare the rules by which you are allowing someone to create an XML file within your workflow, and
- Third, both DTDs and schemas provide methods for restricting, or forcing, the type or format of an element. For example, within the DTD or Schema you can force a date field to be written as 01/05/06 or 1/5/2006.
Differences:
DTDs are better for text-intensive applications, while schemas have several advantages for data-intensive workflows.
Schemas are written in XML and thusly follow the same rules, while DTDs are written in a completely different language.
Examples:
DTD:
<?xml version="1.0" encoding="UTF-8"?>
<!ELEMENT employees (Efirstname, Elastname, Etitle, Ephone, Eemail)>
<!ELEMENT Efirstname (#PCDATA)>
<!ELEMENT Elastname (#PCDATA)>
<!ELEMENT Etitle (#PCDATA)>
<!ELEMENT Ephone (#PCDATA)>
<!ELEMENT Eemail (#PCDATA)>
XSD:
<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:od="urn:schemas-microsoft-com:officedata">
<xsd:element name="dataroot">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="employees" minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
<xsd:attribute name="generated" type="xsd:dateTime"/>
</xsd:complexType>
</xsd:element>
<xsd:element name="employees">
<xsd:annotation>
<xsd:appinfo>
<od:index index-name="PrimaryKey" index-key="Employeeid " primary="yes"
unique="yes" clustered="no"/>
<od:index index-name="Employeeid" index-key="Employeeid " primary="no" unique="no"
clustered="no"/>
</xsd:appinfo>
</xsd:annotation>
<xsd:complexType>
<xsd:sequence>
<xsd:element name="Elastname" minOccurs="0" od:jetType="text"
od:sqlSType="nvarchar">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="50"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="Etitle" minOccurs="0" od:jetType="text" od:sqlSType="nvarchar">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="50"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="Ephone" minOccurs="0" od:jetType="text"
od:sqlSType="nvarchar">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="50"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="Eemail" minOccurs="0" od:jetType="text"
od:sqlSType="nvarchar">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="50"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="Ephoto" minOccurs="0" od:jetType="text"
od:sqlSType="nvarchar">
<xsd:simpleType>
<xsd:restriction base="xsd:string">
<xsd:maxLength value="50"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>
DTD can have only two types of data, the CDATA and the PCDATA. But in a schema you can use all the primitive data type that you use in the programming language and you have the flexibility of defining your own custom data types.
The developer building a schema can create custom data types based on the core data types and by using different operators and modifiers.
One difference is that in a DTD the content model of an element is completely determined by its name, independently of where it appears in the document:
Assuming you want to have
- a
personelement - with a child element called
name - an
nameitself has child elementsfirstandlast.
Like this
<person>
<name>
<first></first>
<last></last>
</name>
</person>
If a city element in the same document also needs to have a child element 'name' the DTD requires that this 'name' element must have child elements first and last as well. Despite the fact that city.name does not require first and last as children.
In contrast, XML Schema allows you to declare child element types locally; you could declare the name child elements for both person and city separately. Thus 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.)
XML DTD
The purpose of a DTD is to define the structure of an XML document. It defines the structure with a list of legal elements:
<!ATTLIST contact type CDATA #IMPLIED>
<!ELEMENT address1 ( #PCDATA)>
<!ELEMENT city ( #PCDATA)>
<!ELEMENT state ( #PCDATA)>
<!ELEMENT zip ( #PCDATA)>
XML Schema
XML Schema enables schema authors to specify that element quantity’s data must
be numeric or, even more specifically, an integer. In the following example I used string:
<xs:element name="note">
<xs:complexType>
<xs:sequence>
<xs:element name="address1" type="xs:string"/>
<xs:element name="city" type="xs:string"/>
<xs:element name="state" type="xs:string"/>
<xs:element name="zip" type="xs:string"/>
</xs:sequence>
</xs:complexType>
DTD indicates the syntax of the XML element
XML Schemas are Microsoft's alternative to DTD for validating XML
When XML first came out, we were told it would solve all our problems: XML will be user-friendly, infinitely extensible, avoid strong-typing, and not require any programming skills. I learnt about DTD's and wrote my own XML parser. 15+ years later, I see that most XML is not user-friendly, and not very extensible (depending on its usage). As soon as some clever clogs hooked up XML to a database I knew that data types were all but inevitable. And, you should see the XSLT (transformation file) I had to work on the other day. If that isn't programming, I don't know what is! Nowadays it's not unusual to see all kinds of problems relating to XML data or interfaces gone bad. I love XML but, it has strayed far from its original altruistic starting point.
The short answer? DTD's have been deprecated in favor of XSD's because an XSD lets you define an XML structure with more precision.
Similarities between XSD and DTD
both specify elements, attributes, nesting, ordering, #occurences
Differences between XSD and DTD
XSD also has data types, (typed) pointers, namespaces, keys and more.... unlike DTD
Moreover though XSD is little verbose its syntax is extension of XML, making it convenient to learn fast.
DTD predates XML and is therefore not valid XML itself. That's probably the biggest reason for XSD's invention.
As many people have mentioned before, XML Schema utilize an XML-based syntax and DTDs have a unique syntax. DTD doesn't support datatypes, which does matter.
Lets see a very simple example in which university has multiple students and each student has two elements "name" and "year". Please note that I have uses "// --> " in my code just for comments.
Now I will write this example both in DTD and in XSD.
DTD
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE university[ // --> university as root element
<!ELEMENT university (student*)> // --> university has * = Multiple students
<!ELEMENT student (name,year)> // --> Student has elements name and year
<!ELEMENT name (#PCDATA)> // --> name as Parsed character data
<!ELEMENT year (#PCDATA)> // --> year as Parsed character data
]>
<university>
<student>
<name>
John Niel //---> I can also use an Integer,not good
</name>
<year>
2000 //---> I can also use a string,not good
</year>
</student>
</university>
XML Schema Definition (XSD)
<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsd:complexType name ="uniType"> //--> complex datatype uniType
<xsd:sequence>
<xsd:element ref="student" maxOccurs="unbounded"/> //--> has unbounded no.of students
</xsd:sequence>
</xsd:complexType>
<xsd:complexType name="stuType"> //--> complex datatype stuType
<xsd:sequence>
<xsd:element ref="name"/> //--> has element name
<xsd:element ref="year"/> //--> has element year
</xsd:sequence>
</xsd:complexType>
<xsd:element name="university" type="uniType"/> //--> university of type UniType
<xsd:element name="student" type="stuType"/> //--> student of type stuType
<xsd:element name="name" type="xsd:string"/> //--> name of datatype string
<xsd:element name="year" type="xsd:integer"/> //--> year of datatype integer
</xsd:schema>
<?xml version="1.0" encoding="UTF-8"?>
<university>
<student>
<name>
John Niel
</name>
<year>
2000 //--> only an Integer value is allowed
</year>
</student>
</university>
Source: Stackoverflow.com