Using variable in XQuery expressions

Question

using Xquery, how can I embed variable in an xquery expression on the following xml document. I have the following xml document

<CD>
        <TITLE>Picture book</TITLE>
        <ARTIST>Simply Red</ARTIST>
        <COUNTRY>EU</COUNTRY>
        <COMPANY>Elektra</COMPANY>
        <PRICE>7.20</PRICE>
        <YEAR>1985</YEAR>
    </CD>
    <CD>
        <TITLE>Red</TITLE>
        <ARTIST>The Communards</ARTIST>
        <COUNTRY>UK</COUNTRY>
        <COMPANY>London</COMPANY>
        <PRICE>7.80</PRICE>
        <YEAR>1987</YEAR>
    </CD>
    <CD>
        <TITLE>Unchain my heart</TITLE>
        <ARTIST>Joe Cocker</ARTIST>
        <COUNTRY>USA</COUNTRY>
        <COMPANY>EMI</COMPANY>
        <PRICE>8.20</PRICE>
        <YEAR>1986</YEAR>
    </CD>

I need to issue a query to return all the CDs with YEAR > 1986. I want like to have the components of the "where" clause as variables, I mean storing YEAR > 1986 in to three variables and use the variables in the query, here what I have so far

String queryString =
                "declare variable $field :=" +"YEAR"+";"+
                "declare variable $operator :=" +">"+";"+
                "declare variable $value :=" +"1986"+";"+
                "declare variable $docName as xs:string external;" + sep +
                "for $cat in doc($docName)/*/"+ "CD" +
                 "where $cat/$field $operator $value" +
                "order by $cat/$field" +
                "return $cat";

              XQExpression expression = conn.createExpression();
              expression.bindString(new QName("docName"), filename,
              conn.createAtomicType(XQItemType.XQBASETYPE_STRING));
              results = expression.executeQuery(queryString);
              return results.getSequenceAsString(new Properties());

My query expression isn't perfect, I think I am having trouble in using the variables, any one can help me in solving this please? Thanks

Michael Kay · Accepted Answer · 2012-06-22 07:01:33Z

2

Variables in XQuery (and XSLT/XPath) represent values, not fragments of program text. That is, it's not a macro language - variables don't work by textual substitution.

Using a variable for the value is straightforward: x = $value.

Using a variable for the element name isn't too difficult: *[name()=$n] = $value

But using a variable for the operator isn't possible. When you get to this level it's better to generate the query as a string using string concatenation, and then compile it and execute it. In fact you seem to be generating the query as a string anyway (without worrying about the risk of code injection, I see).

answered Jun 22, 2012 at 7:01

Michael Kay

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4 Comments

Lucy Over a year ago

Thanks for your answer, Yes that is right variable cannot be used for operator. but can I keep the $field and $value variables in the query and specify the operator explicitly? I mean changing the query to look like the following "where $cat/$field"+ ">"+" $value" ? is this possible?

Daniel Haley Over a year ago

@Lourin - I think you'd need to use an extension function like saxon:evaluate() to do that. What processor are you using?

Lucy Over a year ago

I am using xqj , I've just started working on XQuery, can I use saxon:evaluate() in xqj? Thanks

Michael Kay Over a year ago

XQJ is an interface which can be used with many XQuery processors including Saxon. saxon:evaluate() is particular to Saxon, but many XQuery processors have similar extension functions.

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