XQueryPredikaten

XQueryPredicates are an essential part of the XQuery language, used to filter and select specific nodes from an XML document. They function as conditional expressions enclosed within square brackets [], following a path expression. These predicates evaluate to true or false for each node in the context of the path, and only those nodes for which the predicate evaluates to true are returned.

Commonly, predicates use comparison operators like =, !=, <, >, <=, and >= to compare node values or attributes with specific literals or other node values. For instance, /book[@year > 2000] selects all book elements that have a 'year' attribute with a value greater than 2000. Another powerful feature is the use of the position() function, which allows selection based on a node's ordinal position within its parent. For example, /chapter[position()=1] retrieves only the first chapter.

Predicates can also contain other path expressions, forming nested conditions. The 'and' and 'or' operators can combine multiple conditions within a single predicate. This allows for highly specific and nuanced data selection from complex XML structures. Understanding and effectively using XQuery predicates is crucial for efficient XML data manipulation and querying.