XML and Web Services Reference Guide

XML Query

Last updated Jan 1, 2004.

It doesn't take long to realize that there's a big difference between selecting data from a database using SQL and selecting it from an XML file using XPath. For one thing, an XPath statement is just that: one statement. An SQL statement is typically one statement as well, but most databases have ways to combine statements, such as PL/SQL or Transact SQL.

XML Query, combined with XPath, enables you to create a lot of the same functionality in an XML-esque statement. Consider, for a moment, the hardware order file:

<?xml version="1.0"?>
<order orderid="THX1138" customerNumber="3263827">
    <lineitem itemid="C33">
       <item>3/4" Hex Bolt</item>
       <quantity>36</quantity>
       <unitprice currency="dollars">.35</unitprice>
    </lineitem>
    <lineitem itemid="M48">
       <item>Condenser</item>
       <quantity>1</quantity>
       <unitprice currency="dollars">2200</unitprice>
    </lineitem>
    <delivery>Overnight</delivery>
</order>

We can create a simple file that lists the items that were ordered using XML Query:

<orderInformation>
{
for $item in //item
return
    <itemInfo>
        { $item }
    </itemInfo>
}
</orderInformation>

This is a lot like the for-each (or for-next) statement you find in a lot of langauges. For each node that's returned by the XPath statement, //item, that node gets assigned to the $item variable, and gets output as part of the return statement. In this case, you'd wind up with output of:

<orderInformation>
    <itemInfo>
        <item>3/4" Hex Bold</item>
    </itemInfo>
    <itemInfo>
        <item>Condenser</item>
    </itemInfo>
</orderInformation>

XML Query also enables you to do a straight assignment of a series of nodes to a variable, using the LET statment:

<orderInformation>
{
let $item := //item
return
    <itemInfo>
        { $item }
    </itemInfo>
}
</orderInformation>

This would give you:

<orderInformation>
    <itemInfo>
        <item>3/4" Hex Bold</item>
        <item>Condenser</item>
    </itemInfo>
</orderInformation>

You can also add a qualifier to an XML Query statement, in the form of a WHERE clause:

<orderInformation>
{
let $item := //item
where $item[@itemid="M48"]
return
    <itemInfo>
        { $item }
    </itemInfo>
}
</orderInformation>

This restriction makes the output:

<orderInformation>
    <itemInfo>
        <item>Condenser</item>
    </itemInfo>
</orderInformation>

You can also go ahead and combine these clauses to form a FOR-LET-WHERE-RETURN statement, also known as a FLWR statement:

<orderInformation>
{
for $line in //lineitem
let $item := $line/item, $qty := $line/quantity
where $item[@itemid="M48"]
return
    <itemInfo>
        { $quantity }
        { $item }
    </itemInfo>
}
</orderInformation>

Notice that in addition to combining the FOR and LET clauses, we're defining more than one variable in the LET clause. Notice also that the LET clause uses the $line variable directly. We wind up with:

<orderInformation>
    <itemInfo>
        <quantity>1</quantity>
        <item>Condenser</item>
    </itemInfo>
</orderInformation>

This is, of course, just a limited idea of what you can do with XML Query. Check out the resources for more ideas.