Sequences in XQuery
Core data structure of XQuery
Date: 8/25/2009
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Metadata Solutions
Dan McCreary
President
Dan McCreary & Associates
[email protected]
(952) 931-9198
Agenda/Objective
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What are sequences in XQuery?
Why are they so important?
Comparison with RDBMS/SQL
Why are they critical to understanding XQuery?
Common operations on sequences
A Comparison with other List Processing Languages
Examples
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Two Ways to Represent Data
Tables
Trees
• Tabular data assumes that every column has homogeneous data
– Each item has the same data type
• Sequences are collections of any types of data
Note that the third way is a graph or RDF.
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Table Columns Hold Similar Data
string
date decimal
float
blob
integer
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• Columns must all have the same data type that is
predefined in the Data Definition Language (DDL) using
CREATE TABLE statements.
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Compare
• RDBMS:
int
int
int
int
int
int
int
int
str
str
str
str
str
str
str
str
date
date
date
date
date
date
date
date
• General List Processing Languages:
int
str
int
date
date
str
int
str
xml
date
xml
str
int
str
• XQuery:
int
str
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Sequences of XML items
sequence
XQuery is based on considering each group of XML documents
as a sequence of items
A sequences is of type item()*
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Sequences of Heterogeneous Items
sequence
A sequence can hold any number of items and each item can be a
different type of “object”.
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Search Result Example
A sequence of search “hits”
book
blog
web
play
web
• Each document in a “hit” returns an XML
node
• The data in each “hit” can be different
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Sequence Position
item
position
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• Each “item” in a sequence is numbered
• Numbering starts at “1” (not zero)
• You can get the Nth item by adding the
position as a “predicate” (square brackets)
let $third-item := $sequence[3]
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Counting Items
let $seq := (‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, ‘g’)
let $number-of-items := count($seq)
• You can use the count function to count the
number of items in any sequence
• Example:
– $number-of-items would be 7
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Subsequences
let $new-seq := subsequence($seq, $start, $num)
• You can always create a new sequence
using any range of items in any other
sequence
• Two arguments:
– Number to start
– Number of items you want in the new sequence
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Subsequence Operations
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subsequence($seq, 1, 3)
subsequence($seq, 2, 4)
• Note that the second argument is the
number of items returned
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Last()
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Let $last-item := $seq[last()]
• Returns the position of the last item in a
sequence
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FLOWR Expressions on Sequences
for $item in $sequence
let $tmp := $item/path
order by $tmp
where $item/value = ' x '
return
$item
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Common Sequence Functions
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count
subsequence
distinct-values
index-of
insert-before
last
position
remove
reverse
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Arguments
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count($seq as item()*)
distinct-values($seq as item()*)
index-of ($seq as anyAtomicType()*, $targetanyAtomicType())
insert-before($seq as item()*, $position as int, $inserts as item()*)
last ($seq as item()*)
position() as xs:nonNegativeInteger
remove ($seq as item()*, $position as xs:nonNegativeInteger)
reverse ($seq as item()*)
subsequence ($seq as item()*, $start as xs:nonNegativeInteger, $num
as xs:nonNegativeInteger)
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First Sort Then Get Top Items
Always sort first
Then get the top items
let $sorted-seq :=
for $item in $seq
order by $item/value
return
$item
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let $top-items :=
subsequence($sorted-seq, 1, 10)
• It is very common to need to get a sorted
list of items from a large sequence
• The trick is to sort the items first and then
get the first N items
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Exercises
• Create a sample sequences using letters
– let $input := (‘a’, ‘b’, ‘c’)
• Create a simple XQuery to the common
operations on a sequence.
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Thank You!
Please contact me for more information:
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Native XML Databases
Metadata Management
Metadata Registries
Service Oriented Architectures
Business Intelligence and Data Warehouse
Semantic Web
Dan McCreary, President
Dan McCreary & Associates
Metadata Strategy Development
[email protected]
(952) 931-9198
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