Difference between data() and string()

Question

What is the practical difference between the data() function and the string() function in Xquery? It seems they both return the same results when I use them. Can someone give me a simple example of how they are different?

let $dataset := 
  <data-set cool="rad">
    <!--- This is a comment -->
    <pages cool2="rad2" more="i guess">
      <page>
        <title>Puppy</title>
        <content>Puppies are great!</content>        
      </page>
      <page>
        <title>Dogs</title>
        <content>Dogs are grown up!</content>        
      </page>
      <page>
        <title>Puppy</title>
        <content>Puppies are great!</content>        
      </page>
    </pages>
  </data-set>

return $dataset/string()
(: data() returns the same thing as string() :)
(: return $dataset/data() :)

Answer 1

string() will always return a string, whereas data() returns the "typed value". Most of the time this will be xs:string; however, it can also be xs:untypedAtomic or any other atomic type (typically defined in a schema).

There are some unintuitive cases too. For example, getting data() on a document node will return the string value of the node, but as an xs:untypedAtomic. As a general rule, if you want a string, use string() not data().

See the spec for all the details: http://www.w3.org/TR/xpath-functions/#func-data.

And this section has a very good explanation of the difference between typed- and string-data http://www.w3.org/TR/xpath-datamodel/#typed-string-relationships.