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==Relations or tables==
{{Main|Relation (database)|Table (database)}}
In a relational database, a ''[[relation (database)|relation]]'' is a set of [[tuple]]s that have the same [[Attribute (computing)|attributes]]. A tuple usually represents an object and information about that object. Objects are typically physical objects or concepts. A relation is usually described as a [[Table (database)|table]], which is organized into [[Row (database)|rows]] and [[Column (database)|columns]]. All the data referenced by an attribute are in the same [[data domain|domain]] and conform to the same constraints.

The relational model specifies that the tuples of a relation have no specific order and that the tuples, in turn, impose no order on the attributes. Applications access data by specifying queries, which use operations such as ''select'' to identify tuples, ''project'' to identify attributes, and ''join'' to combine relations. Relations can be modified using the ''insert'', ''delete'', and ''update'' operators. New tuples can supply explicit values or be derived from a query. Similarly, queries identify tuples for updating or deleting.

Tuples by definition are unique. If the tuple contains a [[candidate key|candidate]] or primary key then obviously it is unique; however, a primary key need not be defined for a row or record to be a tuple. The definition of a tuple requires that it be unique, but does not require a primary key to be defined. Because a tuple is unique, its attributes by definition constitute a [[superkey]].