Επιλογή και φιλτράρισμα XQuery

XML instance document

We will continue to use this "books.xml" document in the following examples (the same XML file as used in the previous chapters).

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

Select and filter elements

As seen in the previous chapters, we use path expressions or FLWOR expressions to select and filter elements.

Please see 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) Bind a variable to each item returned by an in expression
let
(Optional)
where
(Optional) Set a condition
order by
(Optional) Set the sorting order of the results
return
Specify the content returned in the result

for statement

The for statement binds a variable to each item returned by an in expression. The for statement can generate iteration. Multiple for statements can exist within the same FLWOR expression.

If you need to loop a specified number of times within a for statement, you can usekeywords to ： 

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

Αποτελέσματα: 

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

关键词 at 可用于计算迭代：

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

Αποτελέσματα:

<book>1. Everyday Italian</book>
<book>2. Harry Potter</book>
<book>3. XQuery Kick Start</book>
<book>4. Learning XML</book>

在 for 语句中同样允许多个 in 表达式。请使用逗号来分割每一个 in 表达式：

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

Αποτελέσματα:

<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>

let 语句

let 语句可完成变量分配，并可避免多次重复相同的表达式。let 语句不会导致迭代。

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

Αποτελέσματα:

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

where 语句

where 语句用于为结果设定一个或多个条件（criteria）。

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

order by 语句

order by 语句用于规定结果的排序次序。在这里，我们要根据 category 和 title 来对结果进行排序：

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

Αποτελέσματα:

<title lang="en">Harry Potter</title>
<title lang="el">Κάθε μέρα Ιταλικά</title>
<title lang="en">Learning XML</title>
<title lang="en">XQuery Kick Start</title>

return 语句：

return 语句规定要返回的内容。

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

Αποτελέσματα:

<title lang="el">Κάθε μέρα Ιταλικά</title>
<title lang="en">Harry Potter</title>
<title lang="en">XQuery Kick Start</title>
<title lang="en">Learning XML</title>