• Chapter 1: Getting Started with Stylus Studio®
• Chapter 2: Editing and Querying XML
• Chapter 3: Converting Non-XML Files to XML
• Chapter 4: Working with XSLT
• Chapter 5: Creating XSLT Using the XSLT Mapper
• Chapter 6: Debugging Stylesheets
• Chapter 7: Defining XML Schemas
• Chapter 8: Defining Document Type Definitions
• Chapter 9: Writing XPath Expressions
• Chapter 10: Working with XQuery in Stylus Studio
  • Getting Started with XQuery in Stylus Studio
  • An XQuery Primer
  • Understanding FLWOR Expressions
    • Simple XQuery FLWOR Expressions
    • The Principal Parts of an XQuery FLWOR Expression
    • Other Parts of the XQuery FLWOR Expression
    • Grouping
  • Building an XQuery Using the Mapper
  • Working with the XQuery collection() Function
  • Debugging XQuery Documents
  • Creating an XQuery Scenario
  • Generating XQuery Documentation
  • Using XQuery to Invoke a Web Service
  • Generating Java Code for XQuery
• Chapter 11: Composing Web Service Calls
• Chapter 12: Accessing Relational Data as XML
• Chapter 13: Building XML Pipelines
• Chapter 14: Publishing XML Data
• Chapter 15: Integrating with Third-Party File Systems
• Chapter 16: Extending Stylus Studio
• Chapter 17: The Stylus Studio Java API
• Chapter 18: Stylus Studio GUI Reference

Home > Online Product Documentation > Table of Contents > Grouping

Grouping

If you are used to SQL, then you might have been wondering what the equivalent to its DISTINCT and GROUP BY keywords is in XQuery FLOWR expressions. Well, SQL does not have one.

You can, however, get a fair amount of mileage from the distinct-values() function. Here is a query that groups videos according to who directed them:

              <movies>
               

                 {for $d in distinct-values(//director) return
               

                    <director name="{$d}">
               

                    { for $v in //video[director = $d] return
               

                       <title>{$v/title}</title>
               

                    }
               

                    </director>
               

                 }
               

              </movies>
               

            

This is not an ideal solution: apart from anything else, it depends heavily on the ability of the query processor to optimize the two nested loops to give good performance. But for the time being, this is all there is. This is an area where vendors are very likely to offer extensions to the language as defined by W3C.

Grouping was a notoriously weak point of XSLT 1.0, and the problem has been addressed with considerable success in the 2.0 version of the language. XQuery will likely follow suit.