Using XPath for XML Document Processing
Lecture 5: Understanding & Using XPATH
UFCFR5-15-3 Advanced Topics in
Web
Development II 2021/22
XML structures data into a rather small number of different constructs,
most notably elements and attributes.
The
XML Path Language (XPath)
defines a way how to select parts of XML
documents, so that they can be used for further processing.
XPath's primary use in in
XSL Transformations (XSLT)
and XQuery (XQ).
XPath is a very compact language with a syntax that resembles path
expressions well-known from file systems. These path expressions,
however, are generalized and therefore much more powerful than the
rather simple path expressions in file systems. Because of its use in
different XML technologies, XPath is one of the most important XML core
technologies.
abstract
o
XML is a syntax for trees
o
it defines a way for how trees can be exchanged
o
XML technologies should provide support for working with trees
o
when receiving trees, access to the tree should be easy (DOM)
o
validating trees should be easy (
XSD
)
o
mapping trees should be easy (
XSLT
)
o
querying tree collections should be easy (
XQuery
)
o
XPath is what regular expressions are for text-based information
why
xpath
(1)
o
Different XML technologies need selection
o
XSLT
needs it for selecting parts and manipulating them
o
XSD
needs it for applying identity constraints
o
DOM needs it for extracting parts from an XML tree
o
XQuery needs it for writing XML-oriented queries
o
XPath was created to be reusable
o
XML experts should only learn one selection language
o
this knowledge can be reused when learning new technologies
o
implementations can reuse code libraries
why xpath (2)
o
XSL was designed as the new XML stylesheet language
o
XSL Transformations (XSLT)
transform the input document
o
XSL Formatting Objects (XSL-FO)
is what they will transform it to
o
XSLT was designed to work on arbitrary XML input documents
o
started as a part of XSL (
WD-xsl-19981216
→
WD-xslt-19990421
)
o
for selecting parts of the transformation input, a selection mechanism had to be
provided
o
XPath was turned into a standalone specification
o
started as a part of XSLT (
WD-xslt-19990421
→
WD-xslt-19990709
)
o
reused in a number of other W3C specifications (XSD, DOM)
o
Complete overhaul for XSLT 2.0 and XQuery
o
XPath 2.0
as the core language (current version is
XPath 3.1
)
o
a much larger set of
functions and operators
o
the underlying
data model
which describes the foundation
how
xpath
evolved
XPath in the family of XML technologies
methods for creating
internal and external
links within XML
documents, and
associating metadata
with those links.
a language for transforming XML documents
into other documents
a query language to extract
data from a XML file or any
collection of data that can be
XML-like.
a language for
locating data within
an XML document
XPath is a language for
finding data in an XML
document.
(Core XML technology)
o
XPath operates on an abstract data model
o
a tree derived from the
XML Information Set (XML Infoset)
o
a simplification (another one!) of the underlying XML
o
The Infoset is turned into an
XPath node tree
o
11 infoset item types → 7 XPath tree node types
o
character items are merged into text nodes
o
namespace declarations are no longer visible as attributes
Starting from the Infoset
Things which are
not in the Infoset
o
the order of attributes in a start tag
o
the types of quotes around attribute values
o
character references and entities (ü/ü → ü)
o
And some more …
o
namespace declarations are no longer visible as attributes
o
notations and unexpanded entity references
What is
NOT
in the Infoset
o
XPath evaluates an expression based on a tree
o
Where the tree comes from is out of XPath's scope
o
The result of the evaluation is a selection
o
//img[not(@alt)]
→ select all images which have no alt attribute
o
count(//img)
→ return the number of images
o
/descendant::img[3]/@src
→ return the third image's src URI
o
starts-with(/html/@lang, 'en')
→ test whether the document's
language is english
o
Syntax errors may occur
xpath selectors: tree in / selection out
o
Each location path consists of
Location Steps
o
location steps are separated by /, like path names in file systems
o
Similarities between XPath location paths and file systems
o
nodes in the
XPath tree
have different types
o
the
type and number of nodes selected by one step
o
the
direction in which each step moves
o
additional
filters for selecting specific nodes
o
Differences between XPath location paths and file systems
o
XPaths may return
other data types than nodes
o
XPath provides a
built-in function library
xpath location paths (1): path structure
xpath location paths (2): file system v. xpath location
o
Name tests
o
testing for a particular name
(elements/attributes):
/html/head/title
o
wildcards (testing for any name):
/html/head/*
o
Node type tests
o
text nodes:
text()
o
comment nodes:
comment()
o
any nodes:
node()
o
Processing instruction tests
o
any PI:
processing-instruction()
o
specific PI:
processing-instruction("xml-stylesheet")xpath location paths (3): test for nodes
o
File system paths are one direction only
o
always one level down in the file system hierarchy
o
. and .. are clever directory shortcuts
o
other directions supported by tools (e.g.,
find
)
o
XPath allows steps is different directions
o
the default direction is
child
o
other directions are explicitly specified:
descendant::a
xpath axis (1): getting there
o
Attributes and Namespaces are
not
the children of elements, but …
o
… elements are their attributes' parent!
o
very counter-intuitive
o
very convenient
o
Attributes and Namespaces are always leaves in the node tree
o
Attribute nodes
have
the attribute value as their value
o
Namespace nodes
have
the namespace name (i.e., a URI) as their value
o
Namespace nodes exist because of namespace declarations
o
in the XPath node tree, only the namespace nodes are visible
o
the namespace declaration attributes (
xmlns
) are invisible
o
one namespace declaration potentially creates many namespace nodes
xpath axis (2): axis peculiarities
xpath axis (3)
xpath has thirteen axis
child
parent
descendent
ancestor
descendent-or-self
ancestor-or-self
following-sibling
preceding-sibling
following
preceding
attribute
namespace
self
o
XPath location paths use a simple syntax
o
sequence of location steps, separated by /
o
Each location step uses a simple structure
(
preceding::p[@class="warning"]
)
o
an axis followed by :: (no axis uses the default axis child)
o
a
node test
o
0-n
Predicates
enclosed in []
o
Location paths can be abbreviated
o
child::
can be omitted (default axis)
o
attribute:: can be written as ’
@
’
o
. is an abbreviation for
self::node()
o
.. is an abbreviation for
parent::node()
o
//
is an abbreviation for
/descendant-or-self::node()/
putting it all together
o
Predicates are filters for each location step
o
there can be any number of filters (
0-n
)
o
each filter is applied to each selected node individually
o
Each predicate is an XPath and evaluated as a boolean
o
the context of this evaluation is the node for which the filter is evaluated
o
if the result is a number, it is compared with the
position()
function
(
/descendant::a[5]
)
o
Predicates always reduce the set of selected nodes
o
as corner cases, the set of selected nodes does not change or is empty
o
predicates are used in the majority of non-trivial XPath location paths
location step filters
Location paths are processed in a very simple way.
1.
start with a given context
2.
for each location step, repeat the following steps:
3.
based on the context and the axis, select the nodes on this axis
4.
reduce this selection to the nodes identified by the node test
5.
sequentially apply all filters to each of these nodes
6.
take the remaining node set as the context for the next location step
location path processing
o
XPath is a full expression language
o
any evaluated expression in XSLT is an XPath
o
XPath must be able to calculate operations on non-XML data types
o
XPath uses a very simple data model
o
node sets:
//img[not(@alt)]
o
number:
count(//img)
o
string:
/descendant::img[3]/@src
o
boolean:
starts-with(/html/@lang, 'en')
xpath expressions
o
XPath is used in different technologies
o
XSLT uses XPath as its expression language
o
XSD uses XPath for selecting identity constraint nodes
o
DOM uses XPath as a way to select DOM nodes
o
Depending on the environment, expression must yield certain results
o
for conditionals, a boolean must be returned
o
iterations (in XSLT) only loop over nodes
o
when printing out text, a string must be produce
o
XPath has built-in rules for casting types
o
node set → boolean: empty is false, non-empty is true
o
node → string: take the
string value
(i.e., concatenate all text node
descendants)
o
string → number: interpret as decimal notation (otherwise return NaN)
o
XPaths often return surprising results
(//a[starts-with(@href, https)])
xpath usage
o
XPath has a small library of built-in functions
o
useful for basic XPath-level functions
o
other specs are allowed to extend it (XSLT does it)
o
XPath functions return results of various data types
o
boolean
:
boolean, contains, false, lang, not, starts-with, true
o
number
:
ceiling, count, floor, last, number, position,
round, string-length, sum
o
string
:
concat, local-name, name, namespace-uri, normalize-space,
string, substring, substring-after, substring-before,
translate
o
node set
:
id
xpath functions
o
Functions and location paths are orthogonal
o
each construct may be based on the other
o
it is possible to nest them arbitrarily
o
predicates often contain functions
//a[substring(@href,string-length(@href)-2)='pdf']
o
XPaths can become powerful and complex
o
writing some code or thinking about an XPath?
o
XPaths are more declarative
o
they may be more robust against changes in the XML schema
o
they can be optimized by a smart XPath implementation
using functions
•
abs
acos
add-dayTimeDurations
add-dayTimeDuration-to-date
add-dayTimeDuration-to-dateTime
add-dayTimeDuration-to-time
add-
yearMonthDurations
add-yearMonthDuration-to-date
add-yearMonthDuration-to-dateTime
adjust-dateTime-to-timezone
adjust-date-
to-timezone
adjust-time-to-timezone
analyze-string
asin
atan
atan2
available-environment-variables
avg
•
base64Binary-equal
base-uri
boolean
boolean-equal
boolean-greater-than
boolean-less-than
•
ceiling
codepoint-equal
codepoints-to-string
collection
compare
concat
concatenate
contains
cos
count
current-
date
current-dateTime
current-time
•
data
date-equal
date-greater-than
date-less-than
dateTime
dateTime-equal
dateTime-greater-than
dateTime-less-than
day-
from-date
day-from-dateTime
days-from-duration
dayTimeDuration-greater-than
dayTimeDuration-less-than
deep-equal
default-
collation
distinct-values
divide-dayTimeDuration
divide-dayTimeDuration-by-dayTimeDuration
divide-yearMonthDuration
divide-
yearMonthDuration-by-yearMonthDuration
doc
doc-available
document-uri
duration-equal
•
element-with-id
empty
encode-for-uri
ends-with
environment-variable
error
escape-html-uri
exactly-
one
except
exists
exp
exp10
•
false
filter
floor
fold-left
fold-right
for-each
for-each-pair
format-date
format-dateTime
format-integer
format-
number
format-time
function-arity
function-lookup
function-name
•
gDay-equal
generate-id
gMonthDay-equal
gMonth-equal
gYear-equal
gYearMonth-equal
•
has-children
head
hexBinary-equal
hours-from-dateTime
hours-from-duration
hours-from-time
•
id
idref
implicit-timezone
index-of
innermost
in-scope-prefixes
insert-before
intersect
iri-to-uri
is-same-node
•
lang
last
local-name
local-name-from-QName
log
log10
lower-case
•
matches
max
min
minutes-from-dateTime
minutes-from-duration
minutes-from-time
month-from-date
month-from-dateTime
months-
from-duration
multiply-dayTimeDuration
multiply-yearMonthDuration
•
name
namespace-uri
namespace-uri-for-prefix
namespace-uri-from-QName
nilled
node-after
node-before
node-name
normalize-
space
normalize-unicode
not
NOTATION-equal
number
numeric-add
numeric-divide
numeric-equal
numeric-greater-than
numeric-
integer-divide
numeric-less-than
numeric-mod
numeric-multiply
numeric-subtract
numeric-unary-minus
numeric-unary-plus
•
one-or-more
outermost
•
parse-xml
parse-xml-fragment
path
pi
position
pow
prefix-from-QName
•
QName
QName-equal
•
remove
replace
resolve-QName
resolve-uri
reverse
root
round
round-half-to-even
•
seconds-from-dateTime
seconds-from-duration
seconds-from-time
serialize
sin
sqrt
starts-with
static-base-
uri
string
string-join
string-length
string-to-codepoints
subsequence
substring
substring-after
substring-
before
subtract-dates
subtract-dateTimes
subtract-dayTimeDuration-from-date
subtract-dayTimeDuration-from-dateTime
subtract-
dayTimeDuration-from-time
subtract-dayTimeDurations
subtract-times
subtract-yearMonthDuration-from-date
subtract-
yearMonthDuration-from-dateTime
subtract-yearMonthDurations
sum
•
tail
tan
time-equal
time-greater-than
time-less-than
timezone-from-date
timezone-from-dateTime
timezone-from-
time
to
tokenize
trace
translate
true
•
union
unordered
unparsed-text
unparsed-text-available
unparsed-text-lines
upper-case
uri-collection
•
year-from-date
year-from-dateTime
yearMonthDuration-greater-than
yearMonthDuration-less-than
years-from-duration
•
zero-or-one
xpath 3.0 functions & operators
o
Query languages select and recombine
1.
look up all addresses by post code
2.
For each post code, count the number of addresses
o
XSLT fills in the missing parts (as a programming language)
o
XSLT can construct XML and re-apply Xpath
o
XQuery fills in the missing parts (query-wise)
o
80% of XQuery is XPath (in version 2.0, though)
o
the remaining 20% are bindings, constructors, and glue
xpath selects
o
XPath is a basic tool of the XML toolbox
o
XPath is reused in various XML technologies
o
XPath selects parts of an XML document
o
XPath can do more general things by using expressions
xpath conclusions
XPath with PHP:
https://www.ibm.com/developerworks/library/x-xpathphp/index.html
XPath functions:
http://dh.obdurodon.org/functions.xhtml
XPath Tester & Evaluator:
https://www.freeformatter.com/xpath-tester.html
XPath syntax:
https://msdn.microsoft.com/en-us/library/ms256471(v=vs.110).aspx
resources:
"XML structures data into elements and attributes, with XPath defining a way to select parts for further processing. Learn why XPath is crucial in XML technologies and how it evolved from XSL to become a powerful language with a wider range of functions and operators."
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Presentation Transcript
UFCFR5-15-3 Advanced Topics in Web Development II 2021/22 Lecture 5: Understanding & Using XPATH
abstract XML structures data into a rather small number of different constructs, most notably elements and attributes. The XML Path Language (XPath) defines a way how to select parts of XML documents, so that they can be used for further processing. XPath's primary use in in XSL Transformations (XSLT) and XQuery (XQ). XPath is a very compact language with a syntax that resembles path expressions well-known from file systems. These path expressions, however, are generalized and therefore much more powerful than the rather simple path expressions in file systems. Because of its use in different XML technologies, XPath is one of the most important XML core technologies.
why xpath (1) o XML is a syntax for trees o it defines a way for how trees can be exchanged o XML technologies should provide support for working with trees o when receiving trees, access to the tree should be easy (DOM) o validating trees should be easy (XSD) o mapping trees should be easy (XSLT) o querying tree collections should be easy (XQuery) o XPath is what regular expressions are for text-based information
why xpath (2) o Different XML technologies need selection o XSLT needs it for selecting parts and manipulating them o XSD needs it for applying identity constraints o DOM needs it for extracting parts from an XML tree o XQuery needs it for writing XML-oriented queries o XPath was created to be reusable o XML experts should only learn one selection language o this knowledge can be reused when learning new technologies o implementations can reuse code libraries
how xpath evolved o XSL was designed as the new XML stylesheet language o XSL Transformations (XSLT) transform the input document o XSL Formatting Objects (XSL-FO) is what they will transform it to o XSLT was designed to work on arbitrary XML input documents o started as a part of XSL (WD-xsl-19981216 WD-xslt-19990421) o for selecting parts of the transformation input, a selection mechanism had to be provided o XPath was turned into a standalone specification o started as a part of XSLT (WD-xslt-19990421 WD-xslt-19990709) o reused in a number of other W3C specifications (XSD, DOM) o Complete overhaul for XSLT 2.0 and XQuery o XPath 2.0 as the core language (current version is XPath 3.1) o a much larger set of functions and operators o the underlying data model which describes the foundation
XPath in the family of XML technologies a query language to extract data from a XML file or any collection of data that can be XML-like. methods for creating internal and external links within XML documents, and associating metadata with those links. a language for locating data within an XML document XPath is a language for finding data in an XML document. (Core XML technology) a language for transforming XML documents into other documents
Starting from the Infoset o XPath operates on an abstract data model o a tree derived from the XML Information Set (XML Infoset) o a simplification (another one!) of the underlying XML o The Infoset is turned into an XPath node tree o 11 infoset item types 7 XPath tree node types o character items are merged into text nodes o namespace declarations are no longer visible as attributes
What is NOT in the Infoset Things which are not in the Infoset o the order of attributes in a start tag o the types of quotes around attribute values o character references and entities (ü/ü ) o And some more o namespace declarations are no longer visible as attributes o notations and unexpanded entity references
xpath selectors: tree in / selection out o XPath evaluates an expression based on a tree o Where the tree comes from is out of XPath's scope o The result of the evaluation is a selection o //img[not(@alt)] select all images which have no alt attribute o count(//img) return the number of images o /descendant::img[3]/@src return the third image's src URI o starts-with(/html/@lang, 'en') test whether the document's language is english o Syntax errors may occur
xpath location paths (1): path structure o Each location path consists of Location Steps o location steps are separated by /, like path names in file systems o Similarities between XPath location paths and file systems o nodes in the XPath tree have different types o the type and number of nodes selected by one step o the direction in which each step moves o additional filters for selecting specific nodes o Differences between XPath location paths and file systems o XPaths may return other data types than nodes o XPath provides a built-in function library
xpath location paths (2): file system v. xpath location File System Path: # Selected Nodes: / usr / local / apache / bin / 1 1 1 1 1 XPath: # Selected Nodes: / html / body / table / thead / tr 1 1 1 6 4 12
xpath location paths (3): test for nodes o Name tests o testing for a particular name (elements/attributes): /html/head/title o wildcards (testing for any name): /html/head/* o Node type tests o text nodes: text() o comment nodes: comment() o any nodes: node() o Processing instruction tests o any PI: processing-instruction() o specific PI: processing-instruction("xml-stylesheet")
xpath axis (1): getting there o File system paths are one direction only o always one level down in the file system hierarchy o . and .. are clever directory shortcuts o other directions supported by tools (e.g., find) o XPath allows steps is different directions o the default direction is child o other directions are explicitly specified: descendant::a
xpath axis (2): axis peculiarities o Attributes and Namespaces are not the children of elements, but o elements are their attributes' parent! o very counter-intuitive o very convenient o Attributes and Namespaces are always leaves in the node tree o Attribute nodes have the attribute value as their value o Namespace nodes have the namespace name (i.e., a URI) as their value o Namespace nodes exist because of namespace declarations o in the XPath node tree, only the namespace nodes are visible o the namespace declaration attributes (xmlns) are invisible o one namespace declaration potentially creates many namespace nodes
xpath axis (3) xpath has thirteen axis child parent descendent ancestor descendent-or-self ancestor-or-self following-sibling preceding-sibling following preceding attribute namespace self
putting it all together o XPath location paths use a simple syntax o sequence of location steps, separated by / o Each location step uses a simple structure (preceding::p[@class="warning"]) o an axis followed by :: (no axis uses the default axis child) o a node test o 0-n Predicates enclosed in [] o Location paths can be abbreviated ochild:: can be omitted (default axis) o attribute:: can be written as @ o . is an abbreviation for self::node() o .. is an abbreviation for parent::node() o// is an abbreviation for /descendant-or-self::node()/
location step filters o Predicates are filters for each location step o there can be any number of filters (0-n) o each filter is applied to each selected node individually o Each predicate is an XPath and evaluated as a boolean o the context of this evaluation is the node for which the filter is evaluated o if the result is a number, it is compared with the position() function (/descendant::a[5]) o Predicates always reduce the set of selected nodes o as corner cases, the set of selected nodes does not change or is empty o predicates are used in the majority of non-trivial XPath location paths
location path processing Location paths are processed in a very simple way. 1. start with a given context 2. for each location step, repeat the following steps: 3. based on the context and the axis, select the nodes on this axis 4. reduce this selection to the nodes identified by the node test 5. sequentially apply all filters to each of these nodes 6. take the remaining node set as the context for the next location step
xpath expressions o XPath is a full expression language o any evaluated expression in XSLT is an XPath o XPath must be able to calculate operations on non-XML data types o XPath uses a very simple data model o node sets: //img[not(@alt)] o number: count(//img) o string: /descendant::img[3]/@src o boolean: starts-with(/html/@lang, 'en')
xpath usage o XPath is used in different technologies o XSLT uses XPath as its expression language o XSD uses XPath for selecting identity constraint nodes o DOM uses XPath as a way to select DOM nodes o Depending on the environment, expression must yield certain results o for conditionals, a boolean must be returned o iterations (in XSLT) only loop over nodes o when printing out text, a string must be produce o XPath has built-in rules for casting types o node set boolean: empty is false, non-empty is true o node string: take the string value (i.e., concatenate all text node descendants) o string number: interpret as decimal notation (otherwise return NaN) o XPaths often return surprising results (//a[starts-with(@href, https)])
xpath functions o XPath has a small library of built-in functions o useful for basic XPath-level functions o other specs are allowed to extend it (XSLT does it) o XPath functions return results of various data types o boolean: boolean, contains, false, lang, not, starts-with, true o number: ceiling, count, floor, last, number, position, round, string-length, sum o string: concat, local-name, name, namespace-uri, normalize-space, string, substring, substring-after, substring-before, translate o node set: id
using functions o Functions and location paths are orthogonal o each construct may be based on the other o it is possible to nest them arbitrarily o predicates often contain functions //a[substring(@href,string-length(@href)-2)='pdf'] o XPaths can become powerful and complex o writing some code or thinking about an XPath? o XPaths are more declarative o they may be more robust against changes in the XML schema o they can be optimized by a smart XPath implementation
add-dayTimeDuration-to-date xpath 3.0 functions & operators abs yearMonthDurations to-timezone base64Binary-equal ceiling date current-dateTime data date-equal from-date day-from-dateTime collation distinct-values yearMonthDuration-by-yearMonthDuration element-with-id empty one except exists false filter floor number format-time gDay-equal generate-id has-children head id idref implicit-timezone lang last local-name matches max min from-duration multiply-dayTimeDuration name namespace-uri space normalize-unicode integer-divide numeric-less-than one-or-more outermost parse-xml parse-xml-fragment QName QName-equal remove replace resolve-QName seconds-from-dateTime uri string string-join before subtract-dates dayTimeDuration-from-time yearMonthDuration-from-dateTime tail tan time-equal time to tokenize union unordered unparsed-text year-from-date year-from-dateTime zero-or-one acos add-dayTimeDurations add-yearMonthDuration-to-date adjust-time-to-timezone base-uri codepoint-equal codepoints-to-string current-time date-greater-than add-dayTimeDuration-to-dateTime add-yearMonthDuration-to-dateTime analyze-string asin atan boolean-equal boolean-greater-than collection compare add-dayTimeDuration-to-time adjust-dateTime-to-timezone available-environment-variables avg boolean-less-than concat concatenate contains add- adjust-date- atan2 boolean cos count current- date-less-than dateTime dateTime-equal dateTime-greater-than dayTimeDuration-less-than dateTime-less-than deep-equal divide-yearMonthDuration day- days-from-duration dayTimeDuration-greater-than divide-dayTimeDuration divide-dayTimeDuration-by-dayTimeDuration doc doc-available document-uri encode-for-uri ends-with environment-variable exp10 fold-left fold-right for-each for-each-pair function-arity function-lookup function-name gMonthDay-equal gMonth-equal gYear-equal hexBinary-equal hours-from-dateTime index-of innermost in-scope-prefixes local-name-from-QName log log10 minutes-from-dateTime minutes-from-duration multiply-yearMonthDuration namespace-uri-for-prefix namespace-uri-from-QName not NOTATION-equal number numeric-add numeric-mod numeric-multiply default- divide- duration-equal error escape-html-uri exactly- exp format-date format-dateTime format-integer format- gYearMonth-equal hours-from-duration hours-from-time intersect insert-before iri-to-uri is-same-node lower-case minutes-from-time month-from-date month-from-dateTime months- nilled node-after numeric-equal numeric-unary-minus node-before numeric-greater-than numeric-unary-plus node-name normalize- numeric- numeric-divide numeric-subtract path pi position pow prefix-from-QName reverse seconds-from-time string-to-codepoints subtract-dayTimeDuration-from-date subtract-dayTimeDurations subtract-yearMonthDurations time-greater-than time-less-than trace translate true unparsed-text-available yearMonthDuration-greater-than resolve-uri root round round-half-to-even serialize subsequence substring seconds-from-duration string-length subtract-dateTimes starts-with substring-after subtract-dayTimeDuration-from-dateTime subtract-yearMonthDuration-from-date sin sqrt static-base- substring- subtract- subtract-times subtract- sum timezone-from-date timezone-from-dateTime timezone-from- unparsed-text-lines upper-case uri-collection years-from-duration yearMonthDuration-less-than
xpath selects o Query languages select and recombine 1. look up all addresses by post code 2. For each post code, count the number of addresses o XSLT fills in the missing parts (as a programming language) o XSLT can construct XML and re-apply Xpath o XQuery fills in the missing parts (query-wise) o 80% of XQuery is XPath (in version 2.0, though) o the remaining 20% are bindings, constructors, and glue
xpath conclusions o XPath is a basic tool of the XML toolbox o XPath is reused in various XML technologies o XPath selects parts of an XML document o XPath can do more general things by using expressions
resources: XPath with PHP: https://www.ibm.com/developerworks/library/x-xpathphp/index.html XPath syntax: https://msdn.microsoft.com/en-us/library/ms256471(v=vs.110).aspx XPath functions: http://dh.obdurodon.org/functions.xhtml XPath Tester & Evaluator: https://www.freeformatter.com/xpath-tester.html
