relation featur?s

Реляционная модель

В статье Кодда (1970) были сформулированы основные принципы реляционной модели, которые отделили схему (логическую структуру) БД от способа ее хранения.

• Based on a single concept – a 2 dimensional table with rows & columns called a relation
• each row, or tuple, is collection of related facts
• each column represents an attribute
• A relational database contains one or more relations, or tables.

Свойства реляций

• tuples have no particular order
• ordering of attributes not important
• no 2 rows (or tuples) are identical
• all values belonging to a particular attribute are from the same domain
• attributes are atomic
• attributes may have a null value

Ссылки

• Codd, E. F. (1970). "A relational model of data for large shared data banks". Communications of the ACM, , Vol. 13, No. 6, pp. 377-387. Retrieved from http://www.acm.org/classics/nov95/toc.html Sept. 4, 2004.
• Codd's 12 rules at answers.com