XQuery: Element Constructors, Sequence Operators, Conditional Expressions, Order, Quantifi, Study notes of Database Programming

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INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Xquery

Prof. Alex Brodsky

INFS 740

GMU

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Element constructors

• To create or add to XML documents• Can use straight XML

– Create a student with name and ID

Laurie

Reeves

123456

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Another FLWR Example

Return

a

list

of

students

who

have

taken

at

least

courses

FOR

$p

IN

document(

“students.xml")//student

LET

$b

document(

“courses.xml)//course[student

$p] WHERE

count($b)

RETURN

$p

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Sequence Operators

•^

Comma separated values^ – 1, 2, 3

•^

Optional brackets^ – (1, 2, 3)– (1,2,(3,4)) = (1,2,3,4)

•^

TO^ – binary operator– Changes operands to integers– e.g. 2 to 5 = (2,3,4,5)

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Conditional expressions

• IF, THEN, and ELSE FOR

$h

IN

//item

RETURN

{$h/name,

IF

($h/@type

“Student")

THEN

$h/ID

ELSE

$h/description

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Order in XML

• BEFORE and AFTER

LET $p := //course[1]FOR $e IN //* AFTER ($p//midterm_1)[1] BEFORE

($p//midterm_2)[2]

RETURN $e

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Quantifiers

• Tests for existence of some elements that

satisfy a condition

• Also used to test whether all elements in a

collection satisfy a condition

• XQuery provides for existential (“some”)

and universal (“every”) quantifiers

•^

Key words

satisfies

and

contains

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Quantified expressions

SOME variable IN expr SATISFIES expre.g. Find the names of faculty who teach a

course in which the description containsJava and XML

FOR $b IN

//faculty

WHERE

SOME $p IN $b//course/description

SATISFIES

(contains($p,

“java") AND

contains($p,

“XML"))

RETURN $b/name

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Filtering

•^

“filter” is a function in the XQuery core functionlibrary

•^

Takes two operands, each of which is anexpression

•^

Returns copies of the nodes in the first operandthat are top level nodes in the second operand

•^

If the two operands don’t have a common root,then filter returns an empty list

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Filtering – Diagram

filter(/C, //A | //B)

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Test or modify datatypes

-^

INSTANCEOF – Boolean binary operator, returnstrue if left operand is the same datatype as theright^ $x INSTANCEOF zoonames:animal{5} instance of element(*, xs:integer)

-^

CAST – changes the datatype^ (x DIV y) CAST AS integer

-^

TREAT – treats one datatype as another for agiven expression^ $mypet TREAT AS Cat

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Explicit Data Types

import schema "ipo.xsd"declare namespace ipo="http://www.example.com/IPO"define function check-address( $a as ipo:address? )

as xsd:boolean

{ … }for $x in document(“foo”)//ipo:invoicewhere $x/ipo:shipTo

instance of

ipo:USAddress

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Conformance Features

• Basic XQuery

– Cannot import XML Schemas; use “built-in”

type names only

– But data do have types– Types are checked dynamically

• Schema Import Feature

– Import schemas: give names to element and

attribute types

INFS 740, Prof. A. Brodsky, IT&E, GMU. Based on material by Prof. X.S. Wang, UVM

Conformance Features

• Static Typing Feature

– Use the type rules specified in the Formal

Semantics Document to check for type errorsbefore query execution.