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

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