Chapter Summary
n The Relational Database Management System (RDBMS) has become the dominant data-processing software
in use today, with estimated new licence sales of between US$6 billion and US$10 billion per year (US$25
billion with tools sales included). This software represents the second generation of DBMSs and is based on
the relational data model proposed by E. F. Codd.
n A mathematical relation is a subset of the Cartesian product of two or more sets. In database terms, a relation
is any subset of the Cartesian product of the domains of the attributes. A relation is normally written as a set
of n-tuples, in which each element is chosen from the appropriate domain.
n Relations are physically represented as tables, with the rows corresponding to individual tuples and the
columns to attributes.
n The structure of the relation, with domain specifications and other constraints, is part of the intension of the
database, while the relation with all its tuples written out represents an instance or extension of the database.
n Properties of database relations are: each cell contains exactly one atomic value, attribute names are distinct,
attribute values come from the same domain, attribute order is immaterial, tuple order is immaterial, and there
are no duplicate tuples.
n The degree of a relation is the number of attributes, while the cardinality is the number of tuples. A unary relation
has one attribute, a binary relation has two, a ternary relation has three, and an n-ary relation has n attributes.
n A superkey is an attribute, or set of attributes, that identifies tuples of a relation uniquely, while a candidate
key is a minimal superkey. A primary key is the candidate key chosen for use in identification of tuples.
A relation must always have a primary key. A foreign key is an attribute, or set of attributes, within one
relation that is the candidate key of another relation.
n A null represents a value for an attribute that is unknown at the present time or is not applicable for this tuple.
n Entity integrity is a constraint that states that in a base relation no attribute of a primary key can be null.
Referential integrity states that foreign key values must match a candidate key value of some tuple in the
home relation or be wholly null. Apart from relational integrity, integrity constraints include, required data,
domain, and multiplicity constraints; other integrity constraints are called general constraints.
n A view in the relational model is a virtual or derived relation that is dynamically created from the underlying
base relation(s) when required. Views provide security and allow the designer to customize a user’s
model. Not all views are updatable.
n The Relational Database Management System (RDBMS) has become the dominant data-processing software
in use today, with estimated new licence sales of between US$6 billion and US$10 billion per year (US$25
billion with tools sales included). This software represents the second generation of DBMSs and is based on
the relational data model proposed by E. F. Codd.
n A mathematical relation is a subset of the Cartesian product of two or more sets. In database terms, a relation
is any subset of the Cartesian product of the domains of the attributes. A relation is normally written as a set
of n-tuples, in which each element is chosen from the appropriate domain.
n Relations are physically represented as tables, with the rows corresponding to individual tuples and the
columns to attributes.
n The structure of the relation, with domain specifications and other constraints, is part of the intension of the
database, while the relation with all its tuples written out represents an instance or extension of the database.
n Properties of database relations are: each cell contains exactly one atomic value, attribute names are distinct,
attribute values come from the same domain, attribute order is immaterial, tuple order is immaterial, and there
are no duplicate tuples.
n The degree of a relation is the number of attributes, while the cardinality is the number of tuples. A unary relation
has one attribute, a binary relation has two, a ternary relation has three, and an n-ary relation has n attributes.
n A superkey is an attribute, or set of attributes, that identifies tuples of a relation uniquely, while a candidate
key is a minimal superkey. A primary key is the candidate key chosen for use in identification of tuples.
A relation must always have a primary key. A foreign key is an attribute, or set of attributes, within one
relation that is the candidate key of another relation.
n A null represents a value for an attribute that is unknown at the present time or is not applicable for this tuple.
n Entity integrity is a constraint that states that in a base relation no attribute of a primary key can be null.
Referential integrity states that foreign key values must match a candidate key value of some tuple in the
home relation or be wholly null. Apart from relational integrity, integrity constraints include, required data,
domain, and multiplicity constraints; other integrity constraints are called general constraints.
n A view in the relational model is a virtual or derived relation that is dynamically created from the underlying
base relation(s) when required. Views provide security and allow the designer to customize a user’s
model. Not all views are updatable.
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