XML Order By: Handling Collation and Missing Values

Sorting XML query results requires a clear understanding of how the order by clause processes linguistic rules and absent data. In standard XML query technologies such as XQuery and SQL/XML, the order by clause sorts extracted nodes and atomic values based on specified collation rules for text comparison and provides explicit keywords—such as empty greatest and empty least—to determine whether missing or empty values appear at the beginning or end of the result set.

Collation in XML Sorting

Collation dictates the rules for comparing and sorting character strings, accounting for cultural conventions, case sensitivity, and accents. When sorting string data within an XML query:

• Default Collation: If no collation is explicitly declared, the query processor uses the default collation defined in the query context (often the Unicode Codepoint Collation or the host database’s default collation). Under the Unicode Codepoint Collation, strings are sorted strictly by their binary character code values, which places all uppercase letters before lowercase letters.

• Explicit Collation: XQuery allows developers to specify a collation URI directly within the order by expression using the collation keyword. For example:

  for $item in /catalog/book
  order by $item/title collation "http://www.w3.org/2005/xpath-functions/collation/codepoint"
  return $item

• Linguistic Ordering: Specifying a locale-specific collation (e.g., language- or region-specific URIs) ensures natural language sorting, where accents and case are treated according to specific cultural standards rather than raw code point values.

Handling Missing and Empty Values

In XML structures, an element or attribute may be absent entirely, or it may evaluate to an empty sequence (()). The order by clause handles these missing values using explicit position specifiers:

• empty greatest: Forces missing or empty values to be treated as larger than any other value. In an ascending sort, missing values appear at the end of the results; in a descending sort, they appear at the beginning.
• empty least: Forces missing or empty values to be treated as smaller than any other value. In an ascending sort, missing values appear at the beginning; in a descending sort, they appear at the end.
• Default Behavior: If neither empty greatest nor empty least is specified, the XQuery specification allows the implementation to choose a default. Consequently, explicitly declaring the empty handling modifier is best practice to ensure consistent and deterministic query results across different XML engines.

Example Syntax

A comprehensive XQuery order by clause combining collation, sort direction, and missing value management follows this structure:

for $employee in /company/employees/employee
order by $employee/department 
    collation "http://www.w3.org/2005/xpath-functions/collation/codepoint" 
    descending 
    empty least,
    $employee/lastName ascending
return $employee

In this example, employees without a department element will appear at the very end because the sort is descending and designated as empty least. Secondary sorting on lastName applies standard ascending order to resolve ties among identical departments.