This chapter is from the book
This chapter is from the book
This chapter is from the book
The Structure of XQuery
Having described why XQuery is more or less necessary, let's look at its general makeup. The language consists of these primary areas:
for/let—Analogous to the SQL SELECT and SET statements, the for and let clauses let you define variables or iterate across a range of sequence values that are in turn assigned to a variable.
where—Analogous to the SQL WHERE statement, the where clause provides a set of conditions that filter or limit the initial selection in a for statement.
order-by—Analogous to SORT BY in SQL, order-by provides the ordering constraints on a sequence.
return—Analogous to the SQL RETURN statement, the XQuery return clause uses a custom formatting language to create output. The output does not necessarily have to be XML, although it is optimized to produce XML.
XPath—Most XPath 2.0 functions are supported in XQuery, as is the axis model used to navigate over XML structures (although some data sources might not support all aspects of XPath because of the type of data involved).
Functions—Analogous to SQL stored procedures, you can define functions in XQuery using the XPath language that can be called inline in an XML query.
Namespaces—A feature of XML rather than SQL. The declare namespace function associates a namespace URI with a prefix, crucial for indicating functionality.
Any number of keywords are associated with XQuery, but these broad categories describe different syntaxes depending upon what specifically needs to be done. For instance, here's an example of a full XQuery expression that uses all five of these areas:
declare namespace xs = "http://www.w3.org/2001/XMLSchema"
define function summaryText($char) returns xs:string
{
concat($char/name,' is a ',$char/gender,' ',$char/species)
}
<results>{
let $chars := input()//character[gender = 'Female']
for $char in $chars
where $char/level gt 5
return
<summary health="{$char/health}"> {
attribute level {$char/level},
attribute date {current-dateTime()},
summaryText($char)
}
</summary>
}
</results>
This is a query for pulling out a series of records from a game database. It assumes an initial XML data stream with records corresponding to different characters, as shown in the file characters.xml:
<characters> <character> <name>Aleria</name> <gender>Female</gender> <species>Heroleim</species> <vocation>Bard</vocation> <level>5</level> <health>25</health> </character> <character> <name>Shar</name> <gender>Male</gender> <species>Human</species> <vocation>Merchant</vocation> <level>6</level> <health>28</health> </character> <character> <name>Gite</name> <gender>Female</gender> <species>Aelvar</species> <vocation>Mage</vocation> <level>7</level> <health>18</health> </character> <character> <name>Horukkan</name> <gender>Male</gender> <species>Udrecht</species> <vocation>Warrior</vocation> <level>5</level> <health>32</health> </character> <character> <name>Gounna</name> <gender>Female</gender> <species>Noleim</species> <vocation>Mage</vocation> <level>8</level> <health>31</health> </character> <character> <name>Sheira</name> <gender>Female</gender> <species>Human</species> <vocation>Cleric</vocation> <level>4</level> <health>9</health> </character> <character> <name>Drue</name> <gender>Female</gender> <species>Voleim</species> <vocation>Warrior</vocation> <level>6</level> <health>32</health> </character> <character> <name>Paccu</name> <gender>Male</gender> <species>Human</species> <vocation>Merchant</vocation> <level>5</level> <health>24</health> </character> </characters>
When the query is executed, it returns an XML document showing all female characters that are of greater than fifth level, with a text summary:
<results> <summary date="2002-10-24T14:38:48" health="18" level="7">Gite is a Female Aelvar</summary> <summary date="2002-10-24T14:38:48" health="31" level="8">
Note that this output has been reformatted somewhat to make it more legible. Whitespace is not usually significant in XQuery—without the formatting, the result of the previous query is as follows:
<results><summary date="2002-10-24T14:38:48" health="18" level="7">
Because each of the major areas has its own language and syntax, the best way to understand the full XQuery language is to break it into the code for each type of expression.