XQuery Selecting and Filtering
Selecting and Filtering Elements
As we have seen in the previous chapters, we are selecting and filtering elements with either a Path expression or with a FLWOR expression.
We will use the "books.xml" file in the examples below.
Look at the following FLWOR expression:
for $x in doc("books.xml")/bookstore/book
where $x/price>30
order by $x/title
return $x/title
- 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.
To loop a specific number of times in a for clause, you may use the to keyword:
for $x in (1 to 5)
return <test>{$x}</test>
Result:
<test>1</test>
<test>2</test>
<test>3</test>
<test>4</test>
<test>5</test>
The at keyword can be used to count the iteration:
for $x at $i in doc("books.xml")/bookstore/book/title
return <book>{$i}. {data($x)}</book>
Result:
<book>1. Everyday Italian</book>
<book>2. Harry Potter</book>
<book>3. XQuery Kick Start</book>
<book>4. Learning XML</book>
It is also allowed with more than one in expression in the for clause. Use comma to separate each in expression:
for $x in (10,20), $y in (100,200)
return <test>x={$x} and y={$y}</test>
Result:
<test>x=10 and y=100</test>
<test>x=10 and y=200</test>
<test>x=20 and y=100</test>
<test>x=20 and y=200</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>
Result:
<test>1 2 3 4 5</test>
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. Here we want to order the result by category and title:
for $x in doc("books.xml")/bookstore/book
order by $x/@category, $x/title
return $x/title
Result:
<title lang="en">Harry Potter</title>
<title lang="en">Everyday Italian</title>
<title lang="en">Learning XML</title>
<title lang="en">XQuery Kick Start</title>
The return Clause
The return clause specifies what is to be returned.
for $x in doc("books.xml")/bookstore/book
return $x/title
Result:
<title lang="en">Everyday Italian</title>
<title lang="en">Harry Potter</title>
<title lang="en">XQuery Kick Start</title>
<title lang="en">Learning XML</title>
