Codd defined the relational algebra [E.F. Codd, Communications of the ACM 13 (1970) 377-387; E.F. Codd, Relational completeness of data base sublanguages, in Data Base Systems, R. Rustin, Ed., Prentice-Hall (1972) 65-98] as the algebra with operations projection, join, restriction, union and difference. His projection operator can drop, permute and repeat columns of a relation. This permuting and repeating of columns does not really add expressive power to the relational algebra. Indeed, using the join operation, one can rewrite any relational algebra expression into an equivalent expression where no projection operator permutes or repeats columns. The fragment of the relational algebra known as the semijoin algebra, however, lacks a full join operation. Nevertheless, we show that any semijoin algebra expression can still be simulated in a natural way by a set of expressions where no projection operator permutes or repeats columns.
Mots-clés : database, relational algebra, semijoin algebra, projection
@article{ITA_2009__43_2_179_0,
author = {Leinders, Dirk and Jan Van Den Bussche},
title = {Repetitions and permutations of columns in the semijoin algebra},
journal = {RAIRO - Theoretical Informatics and Applications - Informatique Th\'eorique et Applications},
pages = {179--187},
publisher = {EDP-Sciences},
volume = {43},
number = {2},
year = {2009},
doi = {10.1051/ita:2008023},
mrnumber = {2512253},
zbl = {1166.68014},
language = {en},
url = {http://www.numdam.org/articles/10.1051/ita:2008023/}
}
TY - JOUR AU - Leinders, Dirk AU - Jan Van Den Bussche TI - Repetitions and permutations of columns in the semijoin algebra JO - RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications PY - 2009 SP - 179 EP - 187 VL - 43 IS - 2 PB - EDP-Sciences UR - http://www.numdam.org/articles/10.1051/ita:2008023/ DO - 10.1051/ita:2008023 LA - en ID - ITA_2009__43_2_179_0 ER -
%0 Journal Article %A Leinders, Dirk %A Jan Van Den Bussche %T Repetitions and permutations of columns in the semijoin algebra %J RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications %D 2009 %P 179-187 %V 43 %N 2 %I EDP-Sciences %U http://www.numdam.org/articles/10.1051/ita:2008023/ %R 10.1051/ita:2008023 %G en %F ITA_2009__43_2_179_0
Leinders, Dirk; Jan Van Den Bussche. Repetitions and permutations of columns in the semijoin algebra. RAIRO - Theoretical Informatics and Applications - Informatique Théorique et Applications, Tome 43 (2009) no. 2, pp. 179-187. doi : 10.1051/ita:2008023. http://www.numdam.org/articles/10.1051/ita:2008023/
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