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. CACM June, 1977 Smith, J. M. Smith, D. C. P. data abstraction, relational database, data type, aggregation, database design, data structure, knowledge representation, data definition language 3.65 3.69 3.79 4.29 4.33 4.34 CA770606 JB December 29, 1977 12:33 AM 2155 4 2957 2406 4 2957 2710 4 2957 2715 4 2957 2716 4 2957 2717 4 2957 2718 4 2957 2765 4 2957 2817 4 2957 2888 4 2957 2901 4 2957 2957 4 2957 2959 4 2957 2965 4 2957 3087 4 2957 3154 4 2957 2046 5 2957 2957 5 2957 2957 5 2957 2957 5 2957 3049 5 2957 2956 6 2957 2958 6 2957 2957 6 2957 2960 6 2957