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Topic: existence of database entities
topics >
computer science >
database >
database as a model of reality
Topic:
database entities
Topic:
entities
Topic:
entity-relationship database model
Topic:
facts as relationships between entities
Topic:
fundamental concepts such as type, attributes, relationships are all the same
Topic:
object and value equivalence
Topic:
unique names
Summary
One of Kent's themes is the importance of existence in a database model. More accurately, a database model expresses awareness of things through its data structures and identifiers. Typically existence is represented by a logical record with a unique identifier and a collection of single-valued facts. Existence may also be represented syntactically, by a predicate, by membership in a list, or implicitly. There is an ill-defined boundary between the representation of existence and relationships. (cbb 3/98)
Subtopic: durable, unique
Quote: need database mechanism to announce the existence of an entity [»kentW1_1984] Quote: the existence of all entities, symbols, relationships should be explicitly asserted [»kentW_1978] Quote: objects with qualified names should be entities independent of the qualifier [»kentW_1978]
Subtopic: detect existence
Quote: durability (preserved under change) and distinguishability are important aspects of identity [»kentW6_1991] Quote: database entities which exist, will have single-valued facts associated with them Quote: the extensional set constructor is the ultimate test of identity; cardinality of the set is one [»kentW6_1991]
Subtopic: list test
Quote: define function 'exist' which is turned into a predicate for testing existence [»deapAM2_1984] Quote: system can detect existence of objects (surrogate, representative, thing) and can interrelate them [»kentW_1978]
Subtopic: syntactic test
Quote: list test for existence: if symbol on a list, then it exists [»kentW_1978] Quote: some records correspond one-to-one with entities; the entity exists only if the record exists; referential integrity and primary keys [»kentW1_1985] Quote: define existence by membership in an existence list for some type [»kentW_1978] Quote: existence by membership in list of objects referenced by symbols [»kentW_1978]
Subtopic: existence is subjective
Quote: syntactic checks for existence: a symbol is numeric, alphabetic, fixed length [»kentW_1978] Quote: syntactic test of existence works for numbers, but not for strings, e.g., addresses [»kentW_1978]
Subtopic: existence not necessary
Quote: a computational system expresses awareness of things (via tokens or references), rather than the existence of things [»kentW6_1991] Quote: there are no objective criteria for the entities identified by data analysis; boundary between entity and non-entity is highly subjective [»kentW1_1985] Quote: can't represent a thing by its description; due to its relationships to other things [»kentW_1978]
Quote: distinct entities do not need to be distinguished, e.g., several vacant jobs [»kentW_1978] Quote: lengths, colors etc. can occur in a database without existence being announced
[»kentW1_1984]
Topic: database as a model of reality (25 items)
Topic: database entities (12 items)
Topic: entities (20 items)
Topic: entity-relationship database model (22 items)
Topic: facts as relationships between entities (22 items)
Topic: fundamental concepts such as type, attributes, relationships are all the same (37 items)
Topic: object and value equivalence (60 items)
Topic: unique names
(58 items)