XQuery

XQuery was designed to query XML data.

XQuery is a language for finding and extracting elements and attributes from XML documents.

XQuery uses path expressions to navigate through elements in an XML document.

example: Xpath 
/bookstore/book/title

XQuery uses predicates to limit the extracted data from XML documents.

example:
/bookstore/book[price<30]

/bookstore/book[price>30]/title

The expression above will select all the title elements under the book elements that are under the bookstore element that have a price element with a value that is higher than 30.

FLWOR expression 

The following FLWOR expression will select exactly the same as the path expression above:

for $x in doc("books.xml")/bookstore/book
where $x/price>30
return $x/title

With FLWOR you can sort the result:

for $x in doc("books.xml")/bookstore/book
where $x/price>30
order by $x/title
return $x/title

The for clause selects all book elements under the bookstore element  into a variable called $x.
The where clause selects only book elements with a price element with a value greater than 30.
The order by clause defines the sort-order. Will be sort by the title element.
The return clause specifies what should be returned. Here it returns the title elements.


XQuery is case-sensitive and XQuery elements, attributes, and variables must be valid XML names.

An XQuery string value can be in single or double quotes

XQuery Conditional Expressions

If-Then-Else

for $x in doc("books.xml")/bookstore/book
return if ($x/@category="CHILDREN")
then <child>{data($x/title)}</child>
else <adult>{data($x/title)}</adult>


Comparisons

1. General comparisons: =, !=, <, <=, >, >=

2. Value comparisons: eq, ne, lt, le, gt, ge


The for Clause

The for clause binds a variable to each item returned by the in expression.
The for clause results in iteration. There can be multiple for clauses in the same FLWOR expression.

Example:

for $x in (1 to 5)
return <test>{$x}</test>

The at keyword can be used to count the iteration:

Example::

for $x at $i in doc("books.xml")/bookstore/book/title
return <book>{$i}. {data($x)}</book>

It is also allowed with more than one in expression in the for clause. Use comma to separate each in expression:

Example:

for $x in (10,20), $y in (100,200)
return <test>x={$x} and y={$y}</test>


The let Clause

The let clause allows variable assignments and it avoids repeating the same expression many times. The let clause does not result in iteration.

let $x := (1 to 5)
return <test>{$x}</test>

The where clause is used to specify one or more criteria for the result:

where $x/price>30 and $x/price<100

The order by clause is used to specify the sort order of the result.

for $x in doc("books.xml")/bookstore/book
order by $x/@category, $x/title
return $x/title

The return clause specifies what is to be returned.

for $x in doc("books.xml")/bookstore/book
return $x/title