XML Interview Questions and Answers

Scalable: Since XML is not in a binary format you can create and edit files with anything and it's also easy to debug. XML can be used to efficiently store small amounts of data like configuration files (web.xml, application.xml, strutsconfig. xml etc) to large company wide data with the help of XML stored in the database. 
Fast Access: XML documents benefit from their hierarchical structure. Hierarchical structures are generally faster to access because you can drill down to the section you are interested in. 
Easy to identify and use: XML not only displays the data but also tells you what kind of data you have. The mark up tags identifies and groups the information so that different information can be identified by different application. 
Style ability: XML is style-free and whenever different styles of output are required the same XML can be used with different style-sheets (XSL) to produce output in XHTML, PDF, TEXT, another XML format etc. 
Linkability, in-line usability, universally accepted standard with free/inexpensive tools

No, they both go together one is for describing data while other is for displaying data.

All data is normally parsed in XML but if you want to exclude some elements you will need to put those elements in CDATA.

A DTD provides a list of the elements, attributes, comments, notes, and entities contained in an XML or HTML document and indicate their relationship with each other. 

DTD supports two types of data - CDATA and PCDATA.
Schema means the organization and the structure of a database. Schema supports numeric, Boolean and String data types and suitable for applications that developed in a programming language. Schema supports custom data types. Schema supports encapsulation and inheritance concepts. Schema supports for WEB services, XSLT.

E.g.: An XML schema is a description of XML document. It is expressed in terms of constraints on the structure and content of documents.

XML parser sits in between the XML document and the application who want to use the XML document. Parser exposes set of well defined interfaces which can be used by the application for adding, modifying and deleting the XML document contents. 

There are two standard specifications which are very common and should be followed by a XML parser:- 

DOM: - Document Object Model.
IT is an open standard. The XML file is arranged in a tree fashion. DOM supports random access to the data of XML file. 
DOM is a W3C recommended way for treating XML documents. In DOM we load entire XML document into memory and allows us to manipulate the structure and data of XML document. 

SAX: - Simple API for XML. 
SAX is event driven way for processing XML documents. In DOM we load the whole XML document in to memory and then application manipulates the XML document. But this is not always the best way to process large XML documents which have huge data elements. For instance you only want one element from the whole XML document or you only want to see if the XML is proper which means loading the whole XML in memory will be quiet resource intensive. SAX parsers parse the XML document sequentially and emit events like start and end of the document, elements, text content etc. So applications who are interested in processing these events can register implementations of callback interfaces. 
SAX parser then only sends those event messages which the application has demanded. 

If we do not need all the data from the XML file then SAX approach is much preferred than DOM as DOM can quiet memory intensive. In short if you need large portion of the XML document its better to have DOM. 
With SAX parser you have to write more code than DOM. 
If we want to write the XML in to a file DOM is the efficient way to do it. 
Some time you only need to validate the XML structure and do not want to retrieve any Data for those instances SAX is the right approach. 

XPATH is an XML query language to select specific parts of an XML document. Using XPATH we can address or filter elements and text in a XML document. For instance a simple XPATH expression like "book/price" states find "price" node which are children of "Invoice" node. XSLT is a rule based language used to transform XML documents in to other file formats. XSLT are nothing but generic transformation rules which can be applied to transform XML document to HTML, CS, Rich text etc.