<P> 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 . </P> <P> Tuples by definition are unique . If the tuple contains a 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 . </P> <P> In a relational database, all data are stored and accessed via relations . Relations that store data are called "base relations", and in implementations are called "tables". Other relations do not store data, but are computed by applying relational operations to other relations . These relations are sometimes called "derived relations". In implementations these are called "views" or "queries". Derived relations are convenient in that they act as a single relation, even though they may grab information from several relations . Also, derived relations can be used as an abstraction layer . </P> <P> A domain describes the set of possible values for a given attribute, and can be considered a constraint on the value of the attribute . Mathematically, attaching a domain to an attribute means that any value for the attribute must be an element of the specified set . The character string "ABC", for instance, is not in the integer domain, but the integer value 123 is . Another example of domain describes the possible values for the field "CoinFace" as ("Heads", "Tails"). So, the field "CoinFace" will not accept input values like (0, 1) or (H, T). </P>

Where is data stored in a relational database