Daplex is a query language based on a functional data model  with the same name. The Daplex data model represents data in terms of entities and functions. The Daplex data model is close to the entity-relationship (ER) model with the difference that relationships between entities in Daplex have a logical direction, whereas ER relationships are directionless. Unlike ER entities, relationships, and properties are all represented as functions in Daplex. Also, entity types are defined as functions without arguments returning sets of a built-in type ENTITY. The entity types are organized in a type/subtype hierarchy. Functions represent properties of entities and relationships among entities. Functions also represent derived information. Functions may be set-valued and are invertible. The database is represented as tabulated function extents. Database updates change the function extents.
The Daplex query language has been very influential for many other query languages, both...
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