descendantssuch

Descendantssuch is a conceptual operator in query languages for hierarchical data such as trees and graphs. It returns the set of descendant nodes of a given context node that satisfy a specified predicate. The operator is used to filter subtrees without enumerating nodes manually.

Semantics of descendantssuch vary by language, but common behavior includes returning all proper descendants (not the context node itself) unless an optional includeSelf flag is set. Each descendant is tested against the predicate, and evaluation may be depth-first or breadth-first depending on implementation. The result is typically a flat collection of nodes or a subgraph representing the matching portion of the subtree.

Syntax examples are often expressed in pseudocode. For instance, descendantssuch(root, n => n.type = 'employee' && n.status = 'active') selects all descendant nodes of root that are employees with active status. Some languages adopt a path-like notation such as root.descendantssuch(predicate) or descendantssuch(root, predicate) within a broader query context.

Relation to other concepts places descendantssuch alongside existing subtree filtering mechanisms. It generalizes the descendant axis found in XPath with an explicit predicate and complements recursive queries used in SQL and graph query languages. Performance characteristics depend on subtree size, predicate selectivity, and available indexing or pruning strategies; lazy evaluation can mitigate materialization costs in large trees.

Applications include filtering subtrees in XML/JSON processing, user interface component trees, and hierarchical access-control models, where precise, predicate-driven descent through a structure is required.

See also: descendant axis, has-descendant, recursive query.