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XQuery Select and Filter

In XQuery you can select and filter elements with either a Path expression or with a FLWOR expression.

Let's see with an example.

Selecting and Filtering Elements​

Consider the following example that contains information about a bookstore.

books.xml
<?xml version="1.0" encoding="UTF-8"?>

<bookstore>
	<book category="children">
		<title lang="en">Harry Potter</title>
		<author>J K. Rowling</author>
		<year>2005</year>
		<price>29.99</price>
	</book>
	<book category="web">
		<title lang="en">XQuery Tips</title>
		<author>Tom Nolan</author>
		<year>2022</year>
		<price>39.99</price>
	</book>
	<book category="web">
		<title lang="en">Learn XML</title>
		<author>Tutorial Reference</author>
		<year>2022</year>
		<price>9.99</price>
	</book>
</bookstore>

Using FLWOR Expression​

Consider the following FLWOR expression:

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

Remember that FLOWR expression is defined using:

  • for: (optional) binds a variable to each item returned by the in expression
  • let: (optional)
  • where: (optional) specifies a criteria
  • order by: (optional) specifies the sort-order of the result
  • return: specifies what to return in the result

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.

For example, to loop a specific number of times in a for clause you can use the to keyword:

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

and the result is:

<example>1</example>  
<example>2</example>  
<example>3</example>  
<example>4</example>  
<example>5</example>

For example, you can use the at keyword to count the iteration:

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

and the result is:

<book>1. Harry Potter</book>
<book>2. XQuery Tips</book>
<book>3. Learn XML</book>

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

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

and the result is:

<example>x=10 and y=100</example>
<example>x=10 and y=200</example>
<example>x=20 and y=100</example>
<example>x=20 and y=200</example>

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

and the result:

<example>1 2 3 4 5</example>

The where Clause​

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​

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

For example, you can order the result by category and title:

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

and the result is sorted by the title element:

<title lang="en">Harry Potter</title>
<title lang="en">Learn XML</title>
<title lang="en">XQuery Tips</title>

The return Clause​

The return clause specifies what is to be returned.

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

and the result is:

<title lang="en">Harry Potter</title>
<title lang="en">XQuery Tips</title>
<title lang="en">Learn XML</title>

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