The term relation schema refers to a heading paired with a set of constraints defined in terms of that heading.
#Create relationship in idatabase free#
Thus, an n-ary relation is interpreted, under the Closed-World Assumption, as the extension of some n-adic predicate: all and only those n-tuples whose values, substituted for corresponding free variables in the predicate, yield propositions that hold true, appear in the relation. Codd used the term "relation" in its mathematical sense of a finitary relation, a set of tuples on some set of n sets S 1, S 2, . The term n-tuple refers to a tuple of degree n ( n ≥ 0).Į. The number of attributes constituting a heading is called the degree, which term also applies to tuples and relations.
A relation is thus a heading paired with a body, the heading of the relation being also the heading of each tuple in its body. A set of tuples that all correspond to the same heading is called a body. It follows from the above definitions that to every tuple there corresponds a unique heading, being the set of names from the tuple, paired with the domains from which the tuple's domain elements are taken.
Thus, in some accounts, a tuple is described as a function, mapping names to values.Ī set of attributes in which no two distinct elements have the same name is called a heading. An attribute value is an attribute name paired with an element of that attribute's domain, and a tuple is a set of attribute values in which no two distinct elements have the same name. An attribute is a name paired with a domain (nowadays more commonly referred to as a type or data type). Instead, each element is termed an attribute value. Codd's original definition notwithstanding, and contrary to the usual definition in mathematics, there is no ordering to the elements of the tuples of a relation. Codd, is a set of tuples (d 1, d 2, ., d n), where each element d j is a member of D j, a data domain. In relational database theory, a relation, as originally defined by E. Relation, tuple, and attribute represented as table, row, and column respectively.