Instancesuch

Instancesuch is a term used in theoretical discussions of data querying and knowledge representation to denote an operator that selects individual instances from a class based on a predicate. The concept is often described in speculative or educational contexts to illustrate how filtering by conditions can be expressed at the level of instances rather than entire collections.

Formally, if C is a class and P(x) is a predicate over members of C, then instancesuch(C, P) denotes the set { x in C | P(x) }. Some presentations write it as an operator applied to a class followed by a condition, analogous to mathematical set-builder notation.

Example: in a data model with a class Person, age is a property. instancesuch(Person, age > 18) yields all persons older than 18. In a product catalog, instancesuch(Product, price < 100 and in_stock) yields affordable, available items.

Origin and usage: instancesuch does not correspond to a standard operator in mainstream query languages. It appears in theoretical discussions, design proposals, or as an illustrative construct to compare with filters, predicates, and where clauses. It has not achieved wide adoption in production database syntax.

Relation to other concepts: It is closely related to filter operations, the where clause in SQL, and the subset selection in predicate logic and set-builder notation. Variants discussed in literature include aliasing or combining with multiple predicates, or adapting the syntax to fit specific data models or query languages.

See also: predicate logic, set-builder notation, filter, where clause, knowledge representation.