XML Query Languages XPATH XQUERY

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XML Query LanguagesXPATHXQUERY
Zaki Malik November 11, 2008
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The XPath/XQuery Data Model

• Corresponding to the fundamental relation of
  the relational model is sequence of items.
• An item is either
• A primitive value, e.g., integer or string.
• A node.

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Principal Kinds of Nodes

1. Document nodes represent entire documents.
2. Elements are pieces of a document consisting of
   some opening tag, its matching closing tag (if
   any), and everything in between.
3. Attributes are names that are given values
   inside opening tags.

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Document Nodes

• Formed by doc(URL) or document(URL) (or
  doc(filename) or document(filename)
• Example doc(/usr/class/cs145/bars.xml)
• All XPath (and XQuery) queries refer to a doc
  node, either explicitly or implicitly.

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Example DTD

• lt!DOCTYPE Bars
• lt!ELEMENT BARS (BAR, BEER)gt
• lt!ELEMENT BAR (PRICE)gt
• lt!ATTLIST BAR name IDgt
• lt!ELEMENT PRICE (PCDATA)gt
• lt!ATTLIST PRICE theBeer IDREFgt
• lt!ELEMENT BEER ()gt
• lt!ATTLIST BEER name ID, soldBy IDREFSgt
• gt

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Example Document

• ltBARSgt
• ltBAR name JoesBargt
• ltPRICE theBeer Exportgt2.50lt/PRICEgt
• ltPRICE theBeer Gr.Is.gt3.00lt/PRICEgt
• lt/BARgt
• ltBEER name Export soldBy JoesBar
• SuesBar /gt
• lt/BARSgt

Document node is all of this, plus the header (
lt? xml version ).
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Nodes as Semistructured Data
bars.xml
BARS
name JoesBar
SoldBy
name Export
BEER
BAR
theBeer Gr.Is.
theBeer Export
PRICE
PRICE
Blue document Green element Orange
attribute Purple primitive value
3.00
2.50
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XPATH and XQUERY

• XPATH is a language for describing paths in XML
  documents.
• Really think of the semi-structured data graph
  and its paths.
• The result of the described path is a sequence of
  items.
• Compare with SQL
• SQL is a language for describing relations in
  terms of other relations.
• The result of a query is a relation (bag) made up
  of tuples
• XQUERY is a full query language for XML documents
  with power similar to SQL.

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Path Descriptors

• Simple path descriptors are sequences of tags
  separated by slashes (/).
• The format used is strongly reminiscent of UNIX
  naming conventions.
• Construct the result by starting with just the
  doc node and processing each tag from the left.
• If the descriptor begins with /, then the path
  starts at the root and has those tags, in order.
• If the descriptor begins with //, then the path
  can start anywhere.

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Example /BARS/BAR/PRICE

• ltBARSgt
• ltBAR name JoesBargt
• ltPRICE theBeer Budgt2.50lt/PRICEgt
• ltPRICE theBeer Millergt3.00lt/PRICEgt
• lt/BARgt
• ltBEER name Bud, soldBy JoesBar,
• SuesBar,gt
• lt/BEERgt
• lt/BARSgt

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Example //PRICE

• ltBARSgt
• ltBAR name JoesBargt
• ltPRICE theBeer Budgt2.50lt/PRICEgt
• ltPRICE theBeer Millergt3.00lt/PRICEgt
• lt/BARgt
• ltBEER name Bud, soldBy JoesBar,
• SuesBar,gt
• lt/BEERgt
• lt/BARSgt

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Wild-Card

• A star () in place of a tag represents any one
  tag.
• Example ///PRICE represents all price objects
  at the third level of nesting.

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Example /BARS/

• ltBARSgt
• ltBAR name JoesBargt
• ltPRICE theBeer Budgt2.50lt/PRICEgt
• ltPRICE theBeer Millergt3.00lt/PRICEgt
• lt/BARgt
• ltBEER name Bud, soldBy JoesBar,
• SuesBar,gt
• lt/BEERgt
• lt/BARSgt

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Attributes

• In XPATH, we refer to attributes by prepending _at_
  to their name.
• Attributes of a tag may appear in paths as if
  they were nested within that tag.

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Example /BARS//_at_name

• ltBARSgt
• ltBAR name JoesBargt
• ltPRICE theBeer Budgt2.50lt/PRICEgt
• ltPRICE theBeer Millergt3.00lt/PRICEgt
• lt/BARgt
• ltBEER name Bud, soldBy JoesBar,
• SuesBar,gt
• lt/BEERgt
• lt/BARSgt

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Selection Conditions

• A condition inside may follow a tag.
• If so, then only paths that have that tag and
  also satisfy the condition are included in the
  result of a path expression.

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Example Selection Condition

• /BARS/BAR/PRICEPRICE lt 2.75
• ltBARSgt
• ltBAR name JoesBargt
• ltPRICE theBeer Budgt2.50lt/PRICEgt
• ltPRICE theBeer Millergt3.00lt/PRICEgt
• lt/BARgt

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Example Attribute in Selection

• /BARS/BAR/PRICE_at_theBeer Miller
• ltBARSgt
• ltBAR name JoesBargt
• ltPRICE theBeer Budgt2.50lt/PRICEgt
• ltPRICE theBeer Millergt3.00lt/PRICEgt
• lt/BARgt

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Axes

• In general, path expressions allow us to start at
  the root and execute a sequence of steps to find
  a set of nodes at each step.
• At each step, we may follow any one of several
  axes.
• The default axis is child --- go to any child
  of the current set of nodes.

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Example Axes

• /BARS/BEER is really shorthand for
  /BARS/childBEER .
• _at_ is really shorthand for the attribute axis.
• Thus, /BARS/BEER_at_name Bud is shorthand for
  
• /BARS/BEERattributename Bud

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More Axes

• Some other useful axes are
• parent parent(s) of the current node(s).
• descendant-or-self the current node(s) and
  all descendants.
• Note // is really a shorthand for this axis.
• ancestor, ancestor-or-self, etc.

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XQuery

• XQuery extends XPath to a query language that has
  power similar to SQL.
• Uses the same sequence-of-items data model as
  XPath.
• XQuery is an expression language.
• Like relational algebra --- any XQuery expression
  can be an argument of any other XQuery expression.

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FLWR Expressions

• The most important form of XQuery expressions
  involves for-, let-, where-, return- (FLWR)
  clauses.
• A qurey begins with one or more for and/or let
  clauses.
• The fors and lets can be interspersed.
• Then an optional where clause.
• A single return clause.
• Form
• for variable in expression
• let variable expression
• where condition
• return expression

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Semantics of FLWR Expressions

• Each for creates a loop.
• let produces only a local variable assignment.
• At each iteration of the nested loops, if any,
  evaluate the where clause.
• If the where clause returns TRUE, invoke the
  return clause, and append its value to the
  output.
• So return can be thought of as add to result

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FOR Clauses

• FOR ltvariablegt IN ltpath expressiongt,
• Variables begin with .
• A FOR variable takes on each object in the set
  denoted by the path expression, in turn.
• Whatever follows this FOR is executed once for
  each value of the variable.

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Example FOR

• FOR beer IN /BARS/BEER/_at_name
• RETURN ltBEERNAMEgtbeerlt/BEERNAMEgt
• beer ranges over the name attributes of all
  beers in our example document.
• Result is a list of tagged names, like
  ltBEERNAMEgtBudlt/BEERNAMEgt ltBEERNAMEgtMillerlt/BEERNAM
  Egt

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LET Clauses

• LET ltvariablegt ltpath expressiongt,
• Value of the variable becomes the set of objects
  defined by the path expression.
• Note LET does not cause iteration FOR does.

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Example LET

• LET beers /BARS/BEER/_at_name
• RETURN ltBEERNAMESgtbeerslt/BEERNAMESgt
• Returns one object with all the names of the
  beers, like
• ltBEERNAMESgtBud, Miller,lt/BEERNAMESgt

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Order-By Clauses

• FLWR is really FLWOR an order-by clause can
  precede the return.
• Form order by ltexpressiongt
• With optional ascending or descending.
• The expression is evaluated for each assignment
  to variables.
• Determines placement in output sequence.

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Example Order-By

• List all prices for Export, lowest price first.
• let d document(bars.xml)
• for p in
• d/BARS/BAR/PRICE_at_theBeerExport
• order by p
• return p

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Following IDREFs

• XQUERY (but not XPATH) allows us to use paths
  that follow attributes that are IDREFs.
• If x denotes a set of IDREFs, then x gty
  denotes all the objects with tag y whose IDs
  are one of these IDREFs.

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Example

• Find all the beer objects where the beer is sold
  by Joes Bar for less than 3.00.
• Strategy
• beer will for-loop over all beer objects.
• For each beer, let joe be either the Joes-Bar
  object, if Joe sells the beer, or the empty set
  of bar objects.
• Test whether joe sells the beer for lt 3.00.

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Example The Query

• FOR beer IN /BARS/BEER
• LET joe beer/_at_soldBygtBAR_at_nameJoesBar
• LET joePrice joe/PRICE_at_theBeerbeer/_at_name
• WHERE joePrice lt 3.00
• RETURN ltCHEAPBEERgtbeerlt/CHEAPBEERgt

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Aggregations

• XQuery allows the usual aggregations, such as
  sum, count, max, min.
• They take any sequence as argument.
• E.g. find bars where all beers are under 5.
• let bars doc(bars.xml)/BARS
• for price in bars/BAR/PRICE
• where max(price) lt 5
• return bar/BAR/_at_name