XQuery Nested For Return

Question

I don't quite understand how to use the for nested within a return. For example, given

<book year="1994">
    <title>TCP/IP Illustrated</title>
    <author><last>Stevens</last><first>W.</first></author>
    <publisher>Addison-Wesley</publisher>
    <price>65.95</price>
</book>

<book year="1992">
    <title>Advanced Programming in the Unix environment</title>
    <author><last>Stevens</last><first>W.</first></author>
    <publisher>Addison-Wesley</publisher>
    <price>65.95</price>
</book>

I want to return the following:

<price group = "65.95">
    <book year = "1994">
    <book year = "1992">
</price>

But with the code I wrote, I am getting the error Undefined variable $b. I was wondering if someone could show me where I'm going wrong. Thanks! Here is my code:

    for $p in distinct-values(//price)
return<price group ="{$p}">
(
  for $b in //book[price = $p]
    return <book year = "{$b/@year}"></book>
)
</price>

Answer 1

The problem is that everything inside the <price group="... element is interpreted as XML. If you want it to be run as XQuery, you have to wrap it in braces {...}:

for $p in distinct-values(//price)
return <price group="{$p}">{
  for $b in //book[price = $p]
  return <book year="{$b/@year}"/>
}</price>

In you code only {$b/@year} is interpreted as XQuery, while the enclosing FLWOR expression is an XML text node. That's why the query processor complains about an undefined variable.