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XML

Structure

  • XML consists of tags and text
  • Tags come in pairs
    • They must be properly nested
    • Tags are case-sensitive: <FOO>...</foo> does not match
    • Tags are nested arbitrarily
  • XML has only one basic type, which is text
  • Two ways of representing a database:
    1. Elements intermixed
    2. One element following another
  • Consistency of ID and IDREF Attribute Values
    • ID stands for identifier. The values across all IDs must be distinct
    • IDREF stands for identifier reference. If an attribute is declared as IDREF, then ...
      • the associated value must exist as the value of some ID attribute (i.e., no dangling "pointers")
      • IDREFS specifies “several” (0 or more) identifiers
      • IDREFs are a lot like Foreign Keys ...except that IDREFs don’t have data types!
  • We say that an XML document is well-formed if the document (with or without an associated DTD) has proper nesting of tags and the attributes of every element are all unique
  • We say that an XML document x is valid with respect to a DTD D if x conforms to D. That is, if the document x conforms to the regular expression grammar and constraints given by D
  • XML Schema: An extension of DTDs that allows one to impose a schema or type on an XML document
  • Query Languages for XML
    • XPath: Language for navigating through an XML document
    • XQuery: Query language for XML, similar in power to SQL
    • XSLT: Language for extracting information from an XML document and transforming it

Template

  • Start the document with a declaration, surrounded by <?xml... ?>
  • The document starts with a root tag that surrounds nested tags
<?xml version = "1.0" standalone = "yes"?>
  • DTD Structure
    • Document Type Descriptors (DTDs)impose structure on an XML document, much like relation schemas impose a structure on relations
    • The DTD is just a syntactic specification, not a semantic specification
<!DOCTYPE <root tag> [
    <!ELEMENT <name>(<components>)>
    . . . more elements . . .
]>

xml

Item Definition/Example
Specify a Tuple <person>
   <name>First Last</name>
   <tel>(999) 999 9999</tel>
   <email>firstlast@ucsc.edu</email>
</person>
Specify a List <addresses>
   <person> ...</person>
   <person> ...</person>
   <person> ...</person>
   ...
</addresses>
Attributes
  • An (opening) tag may contain attributes. These are typically used to describe the content of an element
  • Attributes cannot be repeated within a tag
<entry><br>
   <wordlanguage="en"> cheese </word>
   <wordlanguage="fr"> fromage</word>
   <wordlanguage="ro"> branza</word>
   <meaning>A food made ... </meaning>
</entry>
  • Another common use for attributes is to express dimension or type
<picture>
   <height dim= "cm"> 2400 </height>
   <width dim= "in"> 96</width>
   <data encoding= "gif"compression= "zip">M05-.+C$@02!G96YEFEC ...</data>
</picture>

The segment of an XML document between an opening and a corresponding closing tag is called an element

xml element

Summary of XML Regular Expressions

Expression Definition
A The tag A occurs
e1, e2 The expression e1 followed by e2
e* 0 or more occurrences of e
e? Optional --0 or 1 occurrences
e+ 1 or more occurrences
e1 | e2 either e1 or e2
(e) grouping, e.g., `<!ELEMENT Address Street, (City