Xpath

1
Query Languages for XML
XPath
XQuery
XSLT

2
The XPath/XQuery Data
Model
Corresponding to the fundamental
“relation” of the relational model is:
sequence of items.
An item is either:
1. A primitive value, e.g., integer or string.
2. A node (defined next).

3
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 names that are given
values inside opening tags.

4
Document Nodes
Formed by doc(URL) or document(URL).
Example: doc(/usr/class/cs145/bars.xml)
All XPath (and XQuery) queries refer to a
doc node, either explicitly or implicitly.
Example: key definitions in XML Schema
have Xpath expressions that refer to the
document described by the schema.

5
DTD for Running Example
<!DOCTYPE BARS [
<!ELEMENT BARS (BAR*, BEER*)>
<!ELEMENT BAR (PRICE+)>
<!ATTLIST BAR name ID #REQUIRED>
<!ELEMENT PRICE (#PCDATA)>
<!ATTLIST PRICE theBeer IDREF
#REQUIRED>
<!ELEMENT BEER EMPTY>
<!ATTLIST BEER name ID #REQUIRED>
<!ATTLIST BEER soldBy IDREFS #IMPLIED>
]>

6
Example Document
<BARS>
<BAR name = ”JoesBar”>
<PRICE theBeer = ”Bud”>2.50</PRICE>
<PRICE theBeer = ”Miller”>3.00</PRICE>
</BAR> …
<BEER name = ”Bud” soldBy = ”JoesBar
SuesBar … ”/> …
</BARS>
An element node
An attribute node
Document node is all of this, plus
the header ( <? xml version… ).

7
Nodes as Semistructured
Data
BARS
PRICEPRICE
BEERBAR
name =
”JoesBar”
theBeer =
”Miller”
theBeer
= ”Bud”
SoldBy
= ”…”
name =
”Bud”
3.002.50
Rose =document
Green = element
Gold = attribute
Purple = primitive
value
bars.xml

8
Paths in XML Documents
XPath is a language for describing
paths in XML documents.
The result of the described path is a
sequence of items.

9
Path Expressions
Simple path expressions are sequences
of slashes (/) and tags, starting with /.
Example: /BARS/BAR/PRICE
Construct the result by starting with just
the doc node and processing each tag
from the left.

10
Evaluating a Path Expression
Assume the first tag is the root.
Processing the doc node by this tag results
in a sequence consisting of only the root
element.
Suppose we have a sequence of items,
and the next tag is X.
For each item that is an element node,
replace the element by the subelements
with tag X.

11
Example: /BARS
<BARS>
</BAR> …
</BARS>
One item, the
BARS element

12
Example: /BARS/BAR
<BARS>
<PRICE theBeer =”Bud”>2.50</PRICE>
</BAR> …
SuesBar …”/> …
</BARS> This BAR element followed by
all the other BAR elements

13
Example: /BARS/BAR/PRICE
<BARS>
</BAR> …
</BARS>
These PRICE elements followed
by the PRICE elements
of all the other bars.

14
Attributes in Paths
Instead of going to subelements with a
given tag, you can go to an attribute of
the elements you already have.
An attribute is indicated by putting @ in
front of its name.

15
Example:
/BARS/BAR/PRICE/@theBeer
<BARS>
</BAR> …
</BARS>
These attributes contribute
”Bud” ”Miller” to the result,
followed by other theBeer
values.

16
Remember: Item Sequences
Until now, all item sequences have
been sequences of elements.
When a path expression ends in an
attribute, the result is typically a
sequence of values of primitive type,
such as strings in the previous
example.

17
Paths that Begin Anywhere
If the path starts from the document
node and begins with //X, then the first
step can begin at the root or any
subelement of the root, as long as the
tag is X.

18
Example: //PRICE
<BARS>
</BAR> …
</BARS>
These PRICE elements and
any other PRICE elements
in the entire document

19
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.

20
Example: /BARS/*
<BARS>
</BAR> …
</BARS>
This BAR element, all other BAR
elements, the BEER element, all
other BEER elements

21
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.

22
Example: Selection Condition
/BARS/BAR/PRICE[. < 2.75]
<BARS>
</BAR> …
The condition that the PRICE be
< $2.75 makes this price but not
the Miller price part of the result.
The current
element.

23
Example: Attribute in Selection
/BARS/BAR/PRICE[@theBeer = ”Miller”]
<BARS>
</BAR> …
Now, this PRICE element
is selected, along with
any other prices for Miller.

24
Axes
In general, path expressions allow us to
start at the root and execute steps to find
a sequence of nodes at each step.
At each step, we may follow any one of
several axes.
The default axis is child:: --- go to all the
children of the current set of nodes.

25
Example: Axes
/BARS/BEER is really shorthand for
/BARS/child::BEER .
@ is really shorthand for the attribute::
axis.
Thus, /BARS/BEER[@name = ”Bud” ] is
shorthand for
/BARS/BEER[attribute::name = ”Bud”]

26
More Axes
Some other useful axes are:
1. parent:: = parent(s) of the current
node(s).
2. descendant-or-self:: = the current node(s)
and all descendants.
Note: // is really shorthand for this axis.
3. ancestor::, ancestor-or-self, etc.
4. self (the dot).

27
XQuery
XQuery extends XPath to a query
language that has power similar to SQL.
Uses the same sequence-of-items data
model.
XQuery is an expression language.
Like relational algebra --- any XQuery
expression can be an argument of any other
XQuery expression.

28
More About Item Sequences
XQuery will sometimes form sequences
of sequences.
All sequences are flattened.
Example: (1 2 () (3 4)) = (1 2 3 4).
Empty
sequence

29
FLWR Expressions
1. One or more for and/or let clauses.
2. Then an optional where clause.
3. A return clause.

30
Semantics of FLWR Expressions
Each for creates a loop.
let produces only a local definition.
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.

31
FOR Clauses
for <variable> in <expression>, . . .
Variables begin with $.
A for-variable takes on each item in the
sequence denoted by the expression, in
turn.
Whatever follows this for is executed
once for each value of the variable.

32
Example: FOR
for $beer in
document(”bars.xml”)/BARS/BEER/@name
return
<BEERNAME> {$beer} </BEERNAME>
$beer ranges over the name attributes of all
beers in our example document.
Result is a sequence of BEERNAME
elements: <BEERNAME>Bud</BEERNAME>
<BEERNAME>Miller</BEERNAME> . . .
“Expand the en-
closed string by
replacing variables
and path exps. by
their values.”
Our example
BARS document

33
Use of Braces
When a variable name like $x, or an
expression, could be text, we need to
surround it by braces to avoid having it
interpreted literally.
Example: <A>$x</A> is an A-element with
value ”$x”, just like <A>foo</A> is an A-
element with ”foo” as value.

34
Use of Braces --- (2)
But return $x is unambiguous.
You cannot return an untagged string
without quoting it, as return ”$x”.

35
LET Clauses
let <variable> := <expression>, . . .
Value of the variable becomes the
sequence of items defined by the
expression.
Note let does not cause iteration; for
does.

36
Example: LET
let $d := document(”bars.xml”)
let $beers := $d/BARS/BEER/@name
return
<BEERNAMES> {$beers} </BEERNAMES>
Returns one element with all the names of
the beers, like:
<BEERNAMES>Bud Miller …
</BEERNAMES>

37
Order-By Clauses
FLWR is really FLWOR: an order-by clause
can precede the return.
Form: order by <expression>
With optional ascending or descending.
The expression is evaluated for each
assignment to variables.
Determines placement in output sequence.

38
Example: Order-By
List all prices for Bud, lowest first.
let $d := document(”bars.xml”)
for $p in
$d/BARS/BAR/PRICE[@theBeer=”Bud”
]
order by $p
return $p
Generates bindings
for $p to PRICE
elements.
Order those bindings
by the values inside
the elements (auto-
matic coersion).Each binding is evaluated
for the output. The
result is a sequence of
PRICE elements.

39
Aside: SQL ORDER BY
SQL works the same way; it’s the result
of the FROM and WHERE that get
ordered, not the output.
Example: Using R(a,b),
SELECT b FROM R
WHERE b > 10
ORDER BY a; R tuples with b>10
are ordered by their
a-values.
Then, the b-values
are extracted from these
tuples and printed in the
same order.

40
Predicates
Normally, conditions imply existential
quantification.
Example: /BARS/BAR[@name] means
“all the bars that have a name.”
Example: /BARS/BEER[@soldAt =
”JoesBar”] gives the set of beers that are
sold at Joe’s Bar.

41
Example: Comparisons
Let us produce the PRICE elements
(from all bars) for all the beers that are
sold by Joe’s Bar.
The output will be BBP elements with
the names of the bar and beer as
attributes and the price element as a
subelement.

42
Strategy
1. Create a triple for-loop, with variables
ranging over all BEER elements, all
BAR elements, and all PRICE
elements within those BAR elements.
2. Check that the beer is sold at Joe’s
Bar and that the name of the beer and
theBeer in the PRICE element match.
3. Construct the output element.

43
The Query
let $bars = doc(”bars.xml”)/BARS
for $beer in $bars/BEER
for $bar in $bars/BAR
for $price in $bar/PRICE
where $beer/@soldAt = ”JoesBar” and
$price/@theBeer = $beer/@name
return <BBP bar = {$bar/@name} beer
= {$beer/@name}>{$price}</BBP>
True if ”JoesBar”
appears anywhere
in the sequence

44
Strict Comparisons
To require that the things being
compared are sequences of only one
element, use the Fortran comparison
operators:
eq, ne, lt, le, gt, ge.
Example: $beer/@soldAt eq ”JoesBar”
is true only if Joe’s is the only bar
selling the beer.

45
Comparison of Elements and
Values
When an element is compared to a
primitive value, the element is treated
as its value, if that value is atomic.
Example:
/BARS/BAR[@name=”JoesBar”]/
PRICE[@theBeer=”Bud”] eq ”2.50”
is true if Joe charges $2.50 for Bud.

46
Comparison of Two Elements
It is insufficient that two elements look alike.
Example:
/BARS/BAR[@name=”JoesBar”]/
PRICE[@theBeer=”Bud”] eq
/BARS/BAR[@name=”SuesBar”]/
PRICE[@theBeer=”Bud”]
is false, even if Joe and Sue charge the same
for Bud.

47
Comparison of Elements – (2)
For elements to be equal, they must be
the same, physically, in the implied
document.
Subtlety: elements are really pointers to
sections of particular documents, not
the text strings appearing in the section.

48
Getting Data From Elements
Suppose we want to compare the
values of elements, rather than their
location in documents.
To extract just the value (e.g., the price
itself) from an element E, use data(E ).

49
Example: data()
Suppose we want to modify the return for
“find the prices of beers at bars that sell a
beer Joe sells” to produce an empty BBP
element with price as one of its attributes.
return <BBP bar = {$bar/@name}
beer = {$beer/@name} price =
{data($price)} />

50
Eliminating Duplicates
Use function distinct-values
applied to a sequence.
Subtlety: this function strips tags away
from elements and compares the string
values.
But it doesn’t restore the tags in the result.

51
Example: All the Distinct Prices
return distinct-values(
let $bars = doc(”bars.xml”)
return $bars/BARS/BAR/PRICE
)
Remember: XQuery is
an expression language.
A query can appear any
place a value can.

52
Effective Boolean Values
The effective boolean value (EBV) of
an expression is:
1. The actual value if the expression is of
type boolean.
2. FALSE if the expression evaluates to 0, ””
[the empty string], or () [the empty
sequence].
3. TRUE otherwise.

53
EBV Examples
1. @name=”JoesBar” has EBV TRUE or
FALSE, depending on whether the
name attribute is ”JoesBar”.
2. /BARS/BAR[@name=”GoldenRail”]
has EBV TRUE if some bar is named
the Golden Rail, and FALSE if there is
no such bar.

54
Boolean Operators
E1 and E2, E1 or E2, not(E ), apply to
any expressions.
Take EBV’s of the expressions first.
Example: not(3 eq 5 or 0) has value
TRUE.
Also: true() and false() are functions
that return values TRUE and FALSE.

55
Branching Expressions
if (E1) then E2 else E3 is evaluated by:
Compute the EBV of E1.
If true, the result is E2; else the result is E3.
Example: the PRICE subelements of
$bar, provided that bar is Joe’s.
if($bar/@name eq ”JoesBar”)
then $bar/PRICE else ()
Empty sequence. Note there
is no if-then expression.

56
Quantifier Expressions
some $x in E1 satisfies E2
1. Evaluate the sequence E1.
2. Let $x (any variable) be each item in
the sequence, and evaluate E2.
3. Return TRUE if E2 has EBV TRUE for
at least one $x.
Analogously:
every $x in E1 satisfies E2

57
Example: Some
The bars that sell at least one beer for
less than $2.
for $bar in
doc(”bars.xml”)/BARS/BAR
where some $p in $bar/PRICE
satisfies $p < 2.00
return $bar/@name
Notice: where $bar/PRICE < 2.00
would work as well.

58
Example: Every
The bars that sell no beer for more than
$5.
for $bar in
doc(”bars.xml”)/BARS/BAR
where every $p in $bar/PRICE
satisfies $p <= 5.00
return $bar/@name

59
Document Order
Comparison by document order: << and
>>.
Example: $d/BARS/BEER[@name=”Bud”]
<< $d/BARS/BEER[@name=”Miller”] is
true iff the Bud element appears before
the Miller element in the document $d.

60
Set Operators
union, intersect, except operate on
sequences of nodes.
Meanings analogous to SQL.
Result eliminates duplicates.
Result appears in document order.

61
XSLT
XSLT (extensible stylesheet language –
transforms ) is another language to
process XML documents.
Originally intended as a presentation
language: transform XML into an HTML
page that could be displayed.
It can also transform XML -> XML, thus
serving as a query language.

62
XSLT Programs
Like XML Schema, an XSLT program is
itself an XML document.
XSLT has a special namespace of tags,
usually indicated by xsl:.

63
Templates
The xsl:template element describes a set
of elements (of the document being
processed) and what should be done with
them.
The form: <xsl:template match = path > …
</xsl:template>
Attribute match gives an
XPath expression describing
how to find the nodes to which
the template applies.

64
Example: BARS Document -> Table
In a running example, we’ll convert the
bars.xml document into an HTML
document that looks like the Sells(bar,
beer, price) relation.
The first template will match the root of
the document and produce the table
without any rows.

65
The Template for the Root
<xsl:template match = ”/”>
<TABLE><TR>
<TH>bar</th><TH>beer</th>
<TH>price</th></tr>
</table>
</xsl:template>
Template
matches
only the
root.
Output of the template is
a table with the attributes
in the header row, no
other rows.
Needs to be fixed. As is,
there is no way to insert
rows.

66
Outline of Strategy
1. Inside the HTML for the table is xsl:apply-
templates to extract data from the
document.
2. From each BAR, use an xsl:variable b to
remember the bar name.
3. xsl:for-each PRICE subelement, generate
a row, using b, and xsl:value-of to extract
the beer name and price.

67
Recursive Use of Templates
An XSLT document usually contains
many templates.
Start by finding the first one that applies to
the root.
Any template can have within it
<xsl:apply-templates/>, which causes the
template-matching to apply recursively
from the current node.

68
Apply-Templates
Attribute select gives an XPath
expression describing the subelements
to which we apply templates.
Example: <xsl:apply-templates select =
”BARS/BAR” /> says to follow all paths
tagged BARS, BAR from the current
node and apply all templates there.

69
Example: Apply-Templates
<xsl:template match = ”/”>
<TABLE><TR>
<TH>bar</th><TH>beer</th>
<TH>price</th></tr>
<xsl:apply-templates select =
”BARS” />
</table>
</xsl:template>

70
Extracting Values
<xsl:value-of select = XPath expression />
produces a value to be placed in the
output.
Example: suppose we are applying a
template at a BAR element and want to put
the bar name into a table.
<xsl:value-of select = ”@name” />

71
Variables
We can declare x to be a variable with
<xsl:variable name = ”x” />.
Example:
<xsl:variable name = ”bar”>
</xsl:variable>
within a template that applies to BAR elements
will set variable bar to the name of that bar.

72
Using Variables
Put a $ in front of the variable name.
Example: <TD>$bar</td>

73
Completing the Table
1. We’ll apply a template at each BAR
element.
2. This template will assign a variable b
the value of the bar, and iterate over
each PRICE child.
3. For each PRICE child, we print a row,
using b, the theBeer attribute, and the
PRICE itself.

74
Iteration
<xsl:for-each select = Xpath expression>
…
</xsl:for-each>
executes the body of the for-each at each
child of the current node that is reached
by the path.

75
The Template for BARS
<xsl:template match = ”BAR”>
<xsl:variable name = ”b”>
</xsl:variable>
<xsl:for-each select = ”PRICE”>
<TR><TD>$b</td><TD>
<xsl:value-of select = ”@theBeer” />
</td><TD>
<xsl:value-of select = ”data(.)” />
</td></tr>
</xsl:for-each>
</xsl:template>
Constructs a bar-
beer-price row.
Iterates over all
PRICE subelements
of the bar.
This
element
A variable
for each
bar

Xpath

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