Projection (Relational Algebra)
Lambert M. Surhone, Mariam T. Tennoe, Susan F. Henssonow
Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In relational algebra, a projection is a unary operation written as pi_{a_1, ...,a_n}( R ) where a1,...,an is a set of attribute names. The result of such projection is defined as the set obtained when the components of the tuple R are restricted to the set {a1,...,an} – it discards (or excludes) the other attributes. In practical terms, it can be roughly thought of as picking a sub-set of all available columns. For example, if the attributes are (name, age), then projection of the relation {(Alice, 5), (Bob, 8)} onto attribute list (age) yields {5,8} – we have discarded the names, and only know what ages...
ISBN: 978-6-1346-0697-4
Издательство:
Книга по требованию
Дата выхода: июль 2011