Document Type Definitions - Database Systems - Lecture Slides, Slides of Database Management Systems (DBMS)

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XML

Document Type Definitions

XML Schema

Well-Formed and Valid XML

• Well-Formed XML allows you to invent your own tags.
• Valid XML conforms to a certain DTD.

Tags

• Tags are normally matched pairs, as … .
• Unmatched tags also allowed, as
• Tags may be nested arbitrarily.
• XML tags are case-sensitive.

Example: Well-Formed XML

Joe’s Bar Bud 2.50 Miller 3.00 …

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A NAME subelement

A BEER subelement

Root tag

Tags surrounding a BEER element

DTD Elements

• The description of an element consists of its name (tag), and a parenthesized description of any nested tags. - Includes order of subtags and their multiplicity.
• Leaves (text elements) have #PCDATA ( Parsed Character DATA ) in place of nested tags.

Example: DTD

]>

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A BARS object has zero or more BAR’s nested within.

A BAR has one NAME and one or more BEER subobjects.

A BEER has a NAME and a NAME and PRICE are text. PRICE.

Example: Element Description

• A name is an optional title (e.g., “Prof.”), a first name, and a last name, in that order, or it is an IP address:

(TITLE?, FIRST, LAST) | IPADDR

)>

Use of DTD’s

1. Set standalone = “no”.
2. Either:

a) Include the DTD as a preamble of the XML document, or b) Follow DOCTYPE and the by SYSTEM and a path to the file where the DTD can be found.

Example: (b)

• Assume the BARS DTD is in file bar.dtd.

Joe’s Bar Bud 2.50 Miller 3.00 …

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Get the DTD from the file bar.dtd

Attributes

• Opening tags in XML can have attributes.
• In a DTD,

declares attributes for element E , along with its datatype.

Example: Attribute Use

• In a document that allows BAR tags, we might see:

Homma’s Sapporo 5.00 ...

ID’s and IDREF’s

• Attributes can be pointers from one object to another. - Compare to HTML’s NAME = ”foo” and HREF = ”#foo”.
• Allows the structure of an XML document to be a general graph, rather than just a tree.

Creating IDREF’s

• To allow elements of type F to refer to another element with an ID attribute, give F an attribute of type IDREF.
• Or, let the attribute have type IDREFS, so the F -element can refer to any number of other elements.

Example: ID’s and IDREF’s

• A new BARS DTD includes both BAR and BEER subelements.
• BARS and BEERS have ID attributes name.
• BARS have SELLS subelements, consisting of a number (the price of one beer) and an IDREF theBeer leading to that beer.
• BEERS have attribute soldBy, which is an IDREFS leading to all the bars that sell it.