CS145 Lecture Notes (5) -- XML Queries and Transformations

• Query languages for XML are very new compared to SQL. Despite emerging standards, they are still in flux to some extent.

• XML query languages are not based on a simple and clear underlying algebra, the way SQL is based on relational algebra.

• Sequence of development (roughly):
  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.

XPath

Some basic constructs (very incomplete list):

/	root element, or separator between steps in path
X	matches element X
*	matches any element
@X	matches attribute X of the current element ("context node")
//	matches all descendants of the current element, including self
[C]	evaluates condition C on the current element
[N]	picks the Nth matching element
Path1 | Path2	union of Path1 and Path2 results
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, if there is one
following-sibling::	matches all later siblings of the current element
descendants::	matches all descendants of the current element
self::	matches the current element

XPath inputs and results

• Technically, XPath queries both operate on and return a sequence of items (see textbook).
• Our example XPath queries operate on a single XML document.
• Sometimes the result sequence can be expressed in XML, but not always.

XPath coverage

• We cover a subset of the language through a series of examples.
• The covered subset is sufficient to write lots of queries and understand the flavor of the language.
• See the readings (especially the optional ones) for many more details.

[[On-Line XPath Demo (Saxon)]]

• Simple path expressions
• Alternatives (union)
• Wildcards
• Operator //
• Selecting attributes
• Conditional paths
• Complex conditions
• Nth element
• Parent and Sibling axes
• Contains() predicate
• Name() function

XQuery

• XQuery is an expression language, so its constructs can be freely mixed and matched.

• A primary construct is the FLWOR expression:

    FOR     Specify documents, set up iterator variables
    LET     Set up other variables, usually for aggregation or common subqueries
    WHERE   Filtering condition
    ORDER   Sort results
    RETURN  What to return
  

• XPath expressions are another important construct, and they can be used within all clauses of FLWOR expressions.

XQuery expression, inputs, and results

• Like XPath, technically queries both operate on and return a sequence of items.
• We can write queries in XML that return XML as follows:
  <Result> { ... the query here ... } </Result>

XQuery coverage

• Like for XPath, we cover a subset of the language through a series of examples.
• The covered subset is sufficient to write lots of queries, but it is a smaller subset than for XPath. The full XQuery language is large and complex.
• See the readings (especially the optional ones) for many more details.

[[On-Line XQuery Demo (Saxon)]]

Constructs covered:

• Basic FLWOR queries
• Existential and universal quantification
• Constructed results
• Aggregation
• Ordering
• Duplicate elimination
• Implicit existential

XSLT

• XSL = "Extensible Stylesheet Language"
• XSLT = "XSL Transformations"

XSLT expression, inputs, and results

• Expressed in XML
• Takes XML as input
• Produces XML as output (including HTML as special case)

Recursive, Rule-Based Transformation

• Match template and replace
• Recursively match templates
• Extract values
• Iteration (for-each)
• Conditionals (if)

XSLT coverage

• Once again, we cover a subset of the language, with only a few examples this time.
• The covered subset is yet smaller than for XQuery, but sufficient to understand the flavor of the language.
• See the readings (especially the optional ones) for many more details.

[[On-Line XSLT Demo (Saxon)]]

Constructs covered:

• Basic template matching
• Recursive template matching
• Attribute-subelement transformations
• Iteration
• Sorting
• Conditionals
• Transformation to HTML