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Introduction
XML: Extensible Markup Language
Defined by the WWW Consortium (W3C)
Originally intended as a document markup language not adatabase language
Documents have tags giving extra information about sections of thedocument
E.g. XML Introduction …
Derived from SGML (Standard Generalized Markup Language), butsimpler to use than SGML
Extensible
, unlike HTML
Users can add new tags, and
separately
specify how the tag should
be handled for display
Goal was (is?) to replace HTML as the language for publishingdocuments on the Web
XML Introduction (Cont.)
The ability to specify new tags, and to create nested tag structuresmade XML a great way to exchange
data
, not just documents.
Much of the use of XML has been in data exchange applications, not as areplacement for HTML
Tags make data (relatively) self-documenting
E.g.
A-
Downtown
500
A-
Johnson
XML Motivation (Cont.)
Earlier generation formats were based on plain text with lineheaders indicating the meaning of fields
Similar in concept to email headers
Does not allow for nested structures, no standard “type” language
Tied too closely to low level document structure (lines, spaces, etc)
Each XML based standard defines what are valid elements, using
XML type specification languages to specify the syntax^
DTD (Document Type Descriptors)
XML Schema
Plus textual descriptions of the semantics
XML allows new tags to be defined as required
However, this may be constrained by DTDs
A wide variety of tools is available for parsing, browsing andquerying XML documents/data
Structure of XML Data
Tag
: label for a section of data
Element
: section of data beginning with <
tagname
> and ending
with matching … ….
Improper nesting
… ….
Formally: every start tag must have a unique matching end tag, thatis in the context of the same parent element.
Every document must have a single top-level element
Motivation for Nesting
Nesting of data is useful in data transfer
Example: elements representing customer-id, customer name, andaddress nested within an order element
Nesting is not supported, or discouraged, in relational databases
With multiple orders, customer name and address are storedredundantly
normalization replaces nested structures in each order by foreign keyinto table storing customer name and address information
Nesting is supported in object-relational databases
But nesting is appropriate when transferring data
External application does not have direct access to data referencedby a foreign key
Structure of XML Data (Cont.)
Mixture of text with sub-elements is legal in XML.
Example:
This account is seldom used any more. A-102 Perryridge400
Useful for document markup, but discouraged for datarepresentation
Attributes Vs. Subelements
Distinction between subelement and attribute
In the context of documents, attributes are part of markup, whilesubelement contents are part of the basic document contents
In the context of data representation, the difference is unclear andmay be confusing
Same information can be represented in two ways
A-101 …
Suggestion: use attributes for identifiers of elements, and usesubelements for contents
More on XML Syntax
Elements without subelements or text content can be abbreviatedby ending the start tag with a /> and deleting the end tag
To store string data that may contain tags, without the tags beinginterpreted as subelements, use CDATA as below
Here, and are treated as just strings
XML Document Schema
Database schemas constrain what information can be stored,and the data types of stored values
XML documents are not required to have an associated schema
However, schemas are very important for XML data exchange
Otherwise, a site cannot automatically interpret data received fromanother site
Two mechanisms for specifying XML schema
Document Type Definition (DTD)
Widely used
XML Schema
Newer, increasing use
Document Type Definition (DTD)
The type of an XML document can be specified using a DTD
DTD constraints structure of XML data
What elements can occur
What attributes can/must an element have
What subelements can/must occur inside each element, and howmany times.
DTD does not constrain data types
All values represented as strings in XML
DTD syntax
Bank DTD
]>
Attribute Specification in DTD
Attribute specification : for each attribute
Name
Type of attribute
CDATA
ID (identifier) or IDREF (ID reference) or IDREFS (multiple IDREFs)
more on this later
Whether
mandatory (#REQUIRED)
has a default value (value),
or neither (#IMPLIED)
Examples
ATTLIST customer
customer-id
ID
# REQUIRED
accounts
IDREFS # REQUIRED
Bank DTD with Attributes
Bank DTD with ID and IDREF attribute types.
… declarations for branch, balance, customer-name,
customer-street and customer-city
]>
XML data with ID and IDREF attributes
Downtown
500
Joe
Monroe
Madison
Mary
Erin
Newark
