Relational model: Difference between revisions

Revision as of 01:48, 25 December 2023

Revision as of 01:58, 25 December 2023

Line 17:

[[File:Relational database terms.svg|thumb|A relation with 5 attributes (its degree) and 4 tuples (its cardinality) can be visualized as a table with 5 columns and 4 rows. However, unlike rows and columns in a table, a relation's attributes and tuples are unordered.]]

A ''relation'' consists of a ''heading'' and a ''body''. The heading defines a [[Set (mathematics)|set]] of ''attributes'', each with a ''name'' and ''data type'' (sometimes called a ''___domain''). The number of attributes in this set is the relation's ''degree'' or ''[[arity]]''. The ''body'' is a set of ''tuples'',. eachA with''tuple'' is a collection of ''n'' elements''values'', correspondingwhere to''n'' is the attributesrelation's degree, and each value in the headingtuple corresponds to a unique attribute.{{refn|group=nb|Despite its name, a ''tuple'' in the relational model is not a mathematical [[tuple]]. In mathematics, the elements of a tuple are ordered. In the relational model, attributes are unordered, so the corresponding elements in a tuple are also unordered.}} The number of tuples in this set is the relation's ''[[cardinality]]''.<ref name="professionals"/>{{rp|17-22}}

Relations are represented by ''relational [[Variable (computer science)|variables]]'' or ''relvars'', which can be reassigned.<ref name="professionals"/>{{rp|22-24}}. A ''[[database]]'' is a collection of relvars.<ref name="professionals"/>{{rp|112-113}}.

In this model, databases follow the ''Information Principle'': At any given time, all information in the database is represented solely by values within tuples, corresponding to declared attributes, in relations identified by relvars.<ref name="professionals"/>{{rp|111}}

=== Constraints ===