Editing Relational database (section)

==Base and derived relations==<!-- This section is linked from Relational database. -->
{{Main|Relvar|View (database)}}
All data are stored and accessed via [[relation (database)|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 "[[View (database)|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]].

===Domain===
{{Main|Data domain}}
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).