Relational model: Difference between revisions

Revision as of 00:44, 25 December 2023

Revision as of 01:14, 25 December 2023

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=== Definitions ===

[[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.]]

[[File:Relational model concepts.png|thumb|360px|Relational model concepts.]]

The basic relational building block is the [[Data ___domain|___domain]] or [[data type]], usually abbreviated nowadays to '''''type'''''. A ''[[tuple]]'' is an unordered [[set (mathematics)|set]] of '''''attribute values'''''. An [[Attribute (computing)|attribute]] is an unordered pair of '''''attribute name''''' and '''''type name'''''. An attribute value is a specific valid value for the type of the attribute. This can be either a scalar value or a more complex type.

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A ''[[relvar]]'' is a named variable of some specific relation type, to which at all times some relation of that type is assigned, though the relation may contain zero tuples.

The basic principle of the relational model is the [[Information Principle]]: all [[information]] is represented by data values in relations. In accordance with this Principle, a [[relational database]] is a set of relvars and the result of every query is presented as a relation.t

=== Constraints ===