XQuery Selecting and Filtering

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# XQuery Selecting and Filtering

## The XML Example Document

We will use the "books.xml" document in the examples below (same XML file as in the previous chapters).

View the "books.xml" file in your browser.

## 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.

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>

From http://www.w3schools.com (Copyright Refsnes Data)