Restructuring Data: FLWOR Expressions
The XQuery FLWOR expression is similar to a SQL Select statement that has From and Where clauses. FLWOR is pronounced "flower," and is an acronym for the keywords used to introduce each clause (for, let, where, order by, and return).
Here is a FLWOR expression that returns holdings for AMZN:
for $h in collection('holdings')/holdings where $h/stockticker = 'AMZN' order by $h/shares return $hIn the preceding query, the FLWOR expression performs the following functions:
FLWOR expressions are frequently used to combine related information. The possible combinations are generated by using variables in the for clause and using a where clause to filter out combinations that are not useful. This is known as a "join". Consider the following expression:
for $u in collection('users')/users, $h in collection('holdings')/holdings where $u/userid=$h/userid order by $u/lastname, $u/lastname return <holding> { $u/firstname, $u/lastname, $h/stockticker, $h/shares } </holding>This expression finds every pair of users elements and holdings elements whose userid child element has the same value, and then builds a holding element that describes the user and his holdings.
Now, let's look at a FLWOR expression that uses a let clause:
A let clause binds a variable to a sequence, which often contains more than one item. In the preceding query, $h is bound to all of the holdings elements in the collection, and the return clause is evaluated. Note the difference between a for clause and a let clause: a for clause always iterates over a sequence, binding a variable to each item; a let clause simply binds a variable to the entire sequence.
In the preceding expression, the result is
8. In contrast, if you use the following for clause:The result is a sequence of eight numbers:
1 1 1 1 1 1 1 1.In some cases, you might find it useful to combine for and let clauses. In the following expression, these two clauses are combined to produce a result that counts the number of stock holdings for each user.
for $u in collection('users')/users let $h := collection('holdings')/holdings[userid=$u/userid] order by $u/lastname, $u/firstname return <user nstocks="{count($h)}"> { $u/firstname, $u/lastname } </user>XML Reporting for Relational Sources
Many applications need to create rich XML structures from relational sources. For example, Web sites generally create hierarchical displays of the data found in a relational database, and Web messages are often very complex hierarchical structures. For these applications, XQuery can act as an “XML report writer.”
The database tables used in this section are as follows:
id companyname ticker percentagechange annualrevenues location 1 Apple Computer, Inc. AAPL -40.80% 5250 Cupertino 2 Accrue Software, Inc. ACRU -57.60% 4.21 Freemont ...This query creates a portfolio for each user:
<portfolios> { for $u in collection('users')/users order by $u/userid return <portfolio id="{$u/userid}"> <name> <first>{data($u/firstname)}</first> <last>{data($u/lastname)}</last> </name> <stocks> { for $h in collection('holdings')/holdings where $h/userid = $u/userid order by $h/stockticker return <stock> <ticker>{data($h/stockticker)}</ticker> <shares>{data($h/shares)}</shares> </stock> } </stocks> </portfolio> } </portfolios>Here is the result of the preceding query.
<portfolios> <portfolio id="Jonathan"> <name> <first>Jonathan</first> <last>Robie</last> </name> <stocks> <stock> <ticker>AMZN</ticker> <shares>3000</shares> </stock> <stock> <ticker>EBAY</ticker> <shares>4000</shares> </stock> <stock> <ticker>IBM</ticker> <shares>2500</shares> </stock> <stock> <ticker>PRGS</ticker> <shares>23</shares> </stock> </stocks> </portfolio> <portfolio id="Minollo"> <name> <first>Carlo</first> <last>Innocenti</last> </name> <stocks> <stock> <ticker>AMZN</ticker> <shares>3000</shares> </stock> <stock> <ticker>EBAY</ticker> <shares>4000</shares> </stock> <stock> <ticker>LU</ticker> <shares>40000</shares> </stock> <stock> <ticker>PRGS</ticker> <shares>4000000</shares> </stock> </stocks> </portfolio> </portfolios>NOTE: The query that created this XML result uses the data function, which returns only the value of the stockticker column. Without the data function, the value would be surrounded with an element named stockticker, resulting in, for example:
Processing XML and Relational Together
In some applications, you may need to use XML and relational data together. For example, a configuration file or an incoming Web message might provide information needed to parameterize a query. Suppose you have an XML file that contains a request for a particular kind of report, and your query is to produce that report. For example, the following XML file, request.xml, contains a request to show the performance of Jonathan's stocks during the period from 2003-01-01 to 2004-06-01.
<?xml version="1.0"?> <request> <performance> <UserId>Jonathan</UserId> <start>2003-01-01</start> <end>2004-06-01</end> </performance> </request>Here is a query that creates a portfolio for the user specified in a request file, during the requested period:
declare base-uri "file:///c:/programs/examples/JoinXMLToRelational/"; declare variable $request := doc('request.xml')/request; for $user in $request/performance/UserId, $start in $request/performance/start, $end in $request/performance/end return <portfolio UserId="{$user}"> { $request } { for $st in collection('holdings')/holdings, $stats in collection('statistical')/statistical where $st/userid = $user and $stats/ticker = $st/stockticker return <stock> { $stats/companyname } { $st/stockticker } { $st/shares } { $stats/annualrevenues } { let $hist := for $h in collection('historical')/historical where $h/ticker = $st/stockticker and xs:date($h/datetraded) gt xs:date($start) and xs:date($h/datetraded) lt xs:date($end) return $h return <performance> <min>{min($hist/adjustedclose)}</min> <max>{max($hist/adjustedclose)}</max> <daily> { for $h in $hist return <day>{$h/datetraded, $h/adjustedclose }</day> } </daily> </performance> } </stock> } </portfolio>