2957	 Database Abstractions Aggregation	 Aggregation is in troduced as an abstraction which is important in conceptualizing the real world. Aggregation transforms a relationship between objects into a higher-level object. A new data type called aggregation is developed which under certain criteria of well-definedness specifies aggregation abstractions. Relational databases defined as collections of aggregates are structured as a hierarchy on n-ary relations. To main tain well-definedness update operations on such databases must preserve two invariants. Well-defined relations are distinct from relations in third normal form. It is shown that these notions are complementary and both are important in database design. A top-down methodology for database design is described which separates decisions concerning aggregate structure from decisions concerning key identification. It is suggested that aggregate types and other types which support real-world abstractions without in troducing implementation detail should be incorporated into programming languages. data abstraction relational database data type aggregation database design data structure knowledge representation data definition language
