XML XQuery FLWOR

• XQuery FLWOR stands for For-Let-Where-Order by-Return which is a command used in XQuery to retrieve data that matches specific criteria.
• It is a simple and intuitive way to query XML data.
• It allows you to retrieve data from XML documents based on certain conditions.
• XQuery FLWOR statements consist of four clauses that are executed in a specific order.

Syntax

The basic syntax of the XQuery FLWOR command is as follows:

for $var in sequence
let $var := expression
where condition
order by sort order
return expression

Example

Suppose we have the following XML document:

<library>
  <book>
    <title>Introduction to XQuery</title>
    <author>John Smith</author>
    <year>2017</year>
  </book>
  <book>
    <title>XQuery Advanced</title>
    <author>Jane Doe</author>
    <year>2018</year>
  </book>
  <book>
    <title>Learning XSLT</title>
    <author>Tim Johnson</author>
    <year>2016</year>
  </book>
</library>

We can use the FLWOR command to retrieve the titles of the books published after 2016:

for $book in /library/book
where $book/year > 2016
return $book/title

Output

The output of the above example will be:

<title>XQuery Advanced</title>

Explanation

• The first clause of the FLWOR statement is "for" which defines the variable, $book, and the sequence it will iterate over, /library/book.
• The second clause is "where" which filters the items in the sequence based on the condition, $book/year > 2016.
• The fourth clause is "return" which specifies the result of the query, $book/title.

Use

The XQuery FLWOR command is commonly used to retrieve specific data from XML documents based on certain conditions. It is often used in conjunction with other XQuery functions to manipulate and analyze XML data.

Important Points

• The FLWOR statement must contain at least one "for" clause and one "return" clause.
• The order of the clauses is important and they must be executed in the following order: "for", "let", "where", "order by", and "return".
• The FLWOR statement can include multiple "for" clauses to iterate over multiple sequences.

Summary

XQuery FLWOR is a command used in XQuery to retrieve data from XML documents based on specific criteria. It consists of four clauses that are executed in a specific order: "for", "let", "where", and "return". It is commonly used to query XML data and is often used in conjunction with other XQuery functions to manipulate and analyze XML data.