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.
Implementation	Mature - native implementation	Add - on. (Underlying implementation is relational)

Well formed XML

• Single root element
• Matched tags, proper nesting
• Unique attributes within each element