Databases

Relational Databases

• Set of named relations (tables)
• Each relation has a set of named attributes (columns)
• Each tuple (row) has a value for each attribute
• Each attribute has a type (or domain)
• All queries return relations (tables), so it can said to be compositional or closed.

XML Data

• Alternative to relational model to store data
• Standard for data representation and exchange
• Document format similar to HTML, however tags represent content instead of formatting.
• XML consists of tagged elements which can be nested, attributes of elements and the text (values/data)
• XML document can be thought of as a tree with values/text forming the leaves.

Relational vs XML

	Relational	XML
Structure	Tables	Hierarchical/Tree
Schema	Fixed in advance	Flexible, self-describing. e.g. all elements need not have same attributes. (In relational, we would need to have NULL values). Inconsistencies in the structure is NOT a problem.
Queries	Relatively simple	Complicated compared to Relational / SQL.
Ordering	Fundamentally UNORDERED. e.g. Order by clause is required in SQL in case order is desired.	Ordering is implied - e.g. the way the text is laid out in the XML document or in the Stream.
Implementation	Mature - native implementation	Add - on. (Underlying implementation is relational)

Well formed XML

• Single root element
• XML parser is used to validate XML.
• DOM parser - treats XML as a document, SAX parser - treats XML as a stream.
• Matched tags, proper nesting
• Unique attributes within each element

Displaying XML

• Since XML occurs a lot on the internet, a convenient way is required to display XML.
• Use rule based languages such as CSS, XSL to translate XML to HTML.
• XML document is fed to a CSS/XSL interpreter alongwith rules and a HTML document is generated.

Valid XML

• Valid XML should adhere to requirements for well formed XML
• Also should adhere to content-specific specifications : DTD/XSD
• DTD is a grammar-like language for specifying elements, attributes, nesting, ordering, #occurrences
• DTD/XSD offers the benefits of "typing" - can serve as a specification. Useful for data exchange.
• Having no DTD offers flexibility of data.
• DTD also can specify pointer types - an element can have an ID and another element can refer to it using an IDREF.

XSD

• Like DTD, but more comprehensive.
• XSD is written in XML itself.
• Everything in DTD is string, in XSD e.g., we can specify that an attribute should be of a specific type.
• Pointers can be typed, so we can enforce that a reference is made to a particular type and not just global like in DTD's
• Also specify min/max constraints on elements