1). What is a document type definition and what is its purpose in XML? Explain the difference between a well-formed and a valid XML document.
Answer: The purpose of a DTD is to define the legal building blocks of an XML document. It defines the document structure with a list of legal elements. A DTD can be declared inline in your XML document, or as an external reference. Two definitions:
   -).A well-formed file is one that obeys the general XML rules for tags: tags must be properly nested, opening and closing tags must be balanced, and empty tags must end with '/>'.
   -). A valid file is not only well-formed, but it must also conform to a publicly available DTD that specifies which tags it uses, what attributes those tags can contain, and which tags can occur inside which other tags, among other properties.

2). External DTD:
<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">

3). Minimal but Complete XSLT  Stylesheet

<?xml version="1.0"?>

   < xsl:stylesheet version="1.0"

            xmlns:xsl="http://www.w3.org/1999/ XSL/Transform">

< /xsl:stylesheet >

4). Using XSLT, how would you extract a specfic attribute from an element in an XML document?
Ans: Extract attributes from XML data
   <xsl:template match="element-name">
     Attribute Value:
        <xsl:value-of select="@attribute" />
         <xsl:apply-templates/>
    </xsl:template>

5). Templates--Controls which output is created from which input

--" xsl:template element form

--" match attribute contains an Xpath expression (Xpath expression identifies input node set it matches)

--" For each node in the node set, the

template contents (things between xsl:template tags) are instantiated and inserted into the output tree.

6). Attributes

In the DTD, XML element attributes are declared with an ATTLIST declaration. An attribute declaration has the following syntax:

<!ATTLIST element-name attribute-name attribute-type default-value>

The attribute-type can have the following values:

CDATA

The value is character data

(eval|eval|..)

The value must be an enumerated value

ID

The value is an unique id 

IDREF

The value is the id of another element

IDREFS

The value is a list of other ids

NMTOKEN

The value is a valid XML name

NMTOKENS

The value is a list of valid XML names

ENTITY

The value is an entity 

ENTITIES

The value is a list of entities

NOTATION

The value is a name of a notation

xml:

The value is predefined

The attribute-default-value can have the following values:

#DEFAULT value

The attribute has a default value

#REQUIRED

The attribute value must be included in the element

#IMPLIED

The attribute does not have to be included

#FIXED value

The attribute value is fixed

Attribute declaration example

DTD example:
<!ELEMENT square EMPTY>
  <!ATTLIST square width CDATA "0">

XML example:
<square width="100"></square>

Default attribute value

Syntax:
<!ATTLIST element-name attribute-name CDATA "default-value">

DTD example:
<!ATTLIST payment type CDATA "check">

XML example:
<payment type="check">

Implied attribute

Syntax:
<!ATTLIST element-name attribute-name attribute-type #IMPLIED>

DTD example:
<!ATTLIST contact fax CDATA #IMPLIED>

XML example:
<contact fax="555-667788">

Required attribute

Syntax:
<!ATTLIST element-name attribute_name attribute-type #REQUIRED>

DTD example:
<!ATTLIST person number CDATA #REQUIRED>

XML example:
<person number="5677">

Fixed attribute value

Syntax:
<!ATTLIST element-name attribute-name attribute-type #FIXED "value">

DTD example:
<!ATTLIST sender company CDATA #FIXED "Microsoft">


XML example:
<sender company="Microsoft">

Enumerated attribute values

Syntax:
<!ATTLIST element-name attribute-name (eval|eval|..) default-value>

DTD example:
<!ATTLIST payment type (check|cash) "cash">

XML example:
<payment type="check">
or
<payment type="cash">