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	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		15.00