CS145 Lecture Notes -- XML Queries: XPath and XQuery

• Consider a database containing data encoded in XML. Don't worry about documents, namespaces, etc.

• XML query: issued over database of XML, usually returns XML

• Evolution of query languages for XML:

  1. The database research road:
     1. Lore - invented here!
     2. XML-QL - AT&T Research
     3. Quilt - IBM Almaden Research
     4. XQuery - W3C standard

  2. The standards road:
     1. XPath - path expressions and conditions
     2. XSLT - XPath plus transformations and output formatting
     3. XQuery - XPath + full query language
     XPath is also used in XLink and XPointer

XML DTD and sample data for examples

   <!ELEMENT Bookstore (Book | Magazine)*>
   <!ELEMENT Book (Title, Authors, Remark?)>
   <!ATTLIST Book ISBN CDATA #REQUIRED
             Price CDATA #REQUIRED
             Edition CDATA #IMPLIED>
   <!ELEMENT Magazine (Title)>
   <!ATTLIST Magazine Month CDATA #REQUIRED Year CDATA #REQUIRED> 
   <!ELEMENT Title (#PCDATA)>
   <!ELEMENT Authors (Author+)>
   <!ELEMENT Remark (#PCDATA)>
   <!ELEMENT Author (First_Name, Last_Name)>
   <!ELEMENT First_Name (#PCDATA)>
   <!ELEMENT Last_Name (#PCDATA)>

   <?xml version="1.0" standalone="no"?>
   <!DOCTYPE Bookstore SYSTEM "bookstore.dtd">
   <Bookstore>
      <Book ISBN="ISBN-0-13-035300-0" Price="$65" Edition="2nd">
         <Title>A First Course in Database Systems</Title>
         <Authors>
            <Author>
               <First_Name>Jeffrey</First_Name>
               <Last_Name>Ullman</Last_Name>
            </Author>
            <Author>
               <First_Name>Jennifer</First_Name>
               <Last_Name>Widom</Last_Name>
            </Author>
         </Authors>
      </Book>
      <Book ISBN="ISBN-0-13-031995-3" Price="$75">
         <Title>Database Systems: The Complete Book</Title>
         <Authors>
            <Author>
               <First_Name>Hector</First_Name>
               <Last_Name>Garcia-Molina</Last_Name>
            </Author>
            <Author>
               <First_Name>Jeffrey</First_Name>
               <Last_Name>Ullman</Last_Name>
            </Author>
            <Author>
               <First_Name>Jennifer</First_Name>
               <Last_Name>Widom</Last_Name>
            </Author>
         </Authors>
         <Remark>
         Amazon.com says: Buy this book bundled with "A First Course,"
         it's a great deal!
         </Remark>
      </Book>
   </Bookstore>

XPath

Basic constructs (very incomplete list):

/	root element, or separator between steps in path
*	matches any one element name
@X	matches attribute X of the current element
//	matches any descendant of the current element
[C]	evaluates condition C on the current element
[N]	picks the Nth matching element
contains(s1,s2)	returns TRUE if string s1 contains string s2
name()	returns tag of the current element
parent::	matches the parent of the current element
descendants::	matches any descendant of the current element
self::	matches the current element

Important:

• XPath is a "moving target."
• We cover a subset of the language through a series of examples that conform to some interpretation of the moving target.
• See the readings for more details, but don't worry about the details too much.

(Example: all book titles)

(Example: all book or magazine titles)

(Example: all ISBN numbers)

(Example: all books costing < $70)

(Example: all ISBN numbers of books costing < $70)

(Example: all books containing a remark)

(Example: all titles of books costing < $70 where "Ullman" is an author)

(Example: same query using //)

(Example: all second authors anywhere)

(Example: all author last names anywhere)

(Example: all books whose title contains one of its author's last names)

(Example: all magazines where there is a book of the same title)

(Example: all books where there is a different book of the same title)

(Example: all elements whose parent tag is not "Book")

(Example: all books where all Remarks include "great")

XQuery

   FOR     Set up iterator variables (and specify documents)
   LET     Set up other variables, usually for aggregation or common subqueries
   WHERE   Filtering condition
   RETURN  What to return

Important:

• XQuery is even more of a "moving target" than XPath.
• We cover even less of it, again through a series of examples that conform to some interpretation of the moving target.
• See the readings for more details, but really truly don't worry about the details too much.

(Example: all titles of books costing < $70 where "Ullman" is an author)

(Example: all author Last_Name's of books related to databases)

• "FOR $v in Expr" says: Bind $v to the things returned by Expr one at a time, evaluating the rest of the query for each one.
• "LET $v := Expr" says: Bind $v to the list of all things returned by Expr and keep evaluating the rest of the query.

(Example: average price of all database books)

(Example: all database books priced above average over all books)

(Example: titles and prices of all books, sorted by price)

(Example: titles of books where all author first names contain "Je")

• The EVERY-IN construct adds universal quantification.
• There's also a SOME-IN construct for existential quantification, but it's used less frequently since many constructs are implicitly existentially quantified.

(Example: all book pairs with at least one author last name in common)

(Example: all title-author pairs)