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ORACLE DATABASE ADMINISTRATOR
The relational database dictionary
C.J. Date
09.21.2006
Rating: -3.69- (out of 5)
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The following is an excerpt from The Relational Database Dictionary by C.J. Date, ISBN: 0-596-52798-5, Copyright © 2006 C.J. Date. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.
Primary key: A candidate key that has been singled out for special
syntactic treatment for some reason. While a given relvar can
have any number of candidate keys, it can have at most one
primary key. For a given relvar, however, whether some candidate
key is to be chosen as primary, and if so, which one, are
essentially psychological issues, outside the purview of the
relational model. Note: The relational model originally insisted
that base relvars, at least, should always have a primary key. It
also insisted that foreign keys refer to primary keys specifically.
However, there were never any good logical reasons for
these rules, and rules that apply to base relvars but not to
other kinds are more than a little suspect in any case (because
they violate The Principle of Interchangeability); thus, the primary
key notion could be dropped without serious loss. We
mention it here mainly for historical reasons.
Relational model: The formal theory or foundation on which relational
databases in particular and relational technology in general
are based. The relational model is often loosely
characterized as having three aspects: a structural aspect,
which has to do with relations as such; an integrity aspect,
which has to do with candidate and foreign keys; and a manipulative
aspect, which has to do with operators such as join.
More precisely, the relational model consists of the following
components:
- an open-ended collection of scalar types,
including in particular type BOOLEAN;
- a relation type
generator and an intended interpretation for relations of types
generated thereby;
- facilities for defining relation variables
of such generated relation types;
- a relational assignment
operator; and
- an open-ended collection of generic readonly
operators (i.e., relational algebra or relational calculus)
for deriving relations from relations.
Notice the last item in particular;
it's a far too common error to regard the relational model
as consisting of structure only and to overlook the operators,
and yet (as Codd once said) structure without operators is
rather like anatomy without physiology. Note, moreover, that
those operators aren't just meant for writing queries, as many
seem to think; rather, they're for writing expressions, expressions
that serve many purposes (including query but not limited
to it alone). One particularly important purpose is the
formulation of constraints (though in this case, the relational
expression will be just a subexpression of some boolean
expression, frequently though not invariably an invocation of
IS_EMPTY, q.v.). Note: In the interest of physical data independence,
the relational model is deliberately silent on everything
to do with performance.
Table: 1. SQL analog of either a relation or a relvar, as the context
demands. Here are some of the major differences between
tables in SQL and their relational counterparts:
- SQL tables
can contain duplicate rows;
- SQL tables can contain nulls;
- SQL tables have a left-to-right column sequence;
- SQL
tables can have two or more columns with the same name;
- SQL tables can have what are, in effect, columns with no name
at all.
2. More generally, a picture of a relation (on paper, for
example). Note: A confusion between relations and such tabular
pictures probably accounts for the popular misconception
that "relations are flat," or two-dimensional. While it's obviously
true that those pictures are two-dimensional, relations in
general aren't; rather, a relation of degree n is n-dimensional,
in the sense that its tuples correspond to points in some n-dimensional
space (one dimension for each attribute of the
relation in question). One specific consequence of such considerations
is that (again contrary to popular opinion) relations
are perfectly capable of representing so-called
multidimensional data and thereby supporting so-called online
analytical processing (OLAP).
About the author
C.J. Date is an independent author, lecturer and researcher, specializing in relational database technology (a field he helped pioneer). He is best known for his books, especially An Introduction to Database Systems, which has sold nearly 750,000 copies and is used by universities worldwide.
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