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In relational database theory, a relation, as originally defined by E. F. Codd, is a set of tuples (d1, d2, ..., dn), where each element dj is a member of Dj, a data domain. 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. Instead, each element is termed an attribute value. An attribute is a name paired with a domain (nowadays more commonly referred to as a type or data type). 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. Thus, in some accounts, a tuple is described as a function, mapping names to values. A set of attributes in which no two distinct elements have the same name is called a heading. A set of tuples having the same heading is called a body. 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. The number of attributes constituting a heading is called the degree, which term also applies to tuples and relations. The term ''n''-tuple refers to a tuple of degree ''n'' (''n''>=0). E. F. 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, .... ,''Sn''. 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. The term relation schema refers to a heading paired with a set of constraints defined in terms of that heading. A relation can thus be seen as an instantiation of a relation schema if it has the heading of that schema and it satisfies the applicable constraints. Sometimes a relation schema is taken to include a name. A relational database definition (database schema, sometimes referred to as a relation''al'' schema) can thus be thought of as a collection of named relation schemas. In implementations, the domain of each attribute is effectively a data type and a named relation schema is effectively a relation variable or relvar for short (see Relation Variables below). In SQL, a database language for relational databases, relations are represented by tables, where each row of a table represents a single tuple, and where the values of each attribute form a column. == Examples == Below is an example of a relation having three named attributes: 'ID' from the domain of integers, and 'Name' and 'Address' from the domain of strings: A predicate for this relation, using the attribute names to denote free variables, might be "Employee number ''ID'' is known as ''Name'' and lives at ''Address''". Examination of the relation tells us that there are just four tuples for which the predicate holds true. So, for example, employee 102 is known only by that name, Yonezawa Akinori, and does not live anywhere else but in Naha, Okinawa. Also, apart from the four employees shown, there is no other employee who has both a name and an address. Under the definition of body, the tuples of a body do not appear in any particular order - one cannot say "The tuple of 'Murata Makoto' is above the tuple of 'Matsumoto Yukihiro'", nor can one say "The tuple of 'Yonezawa Akinori' is the first tuple." A similar comment applies to the rows of an SQL table. Under the definition of heading, the attributes of an element do not appear in any particular order either, nor, therefore do the elements of a tuple. A similar comment does ''not'' apply here to SQL, which does define an ordering to the columns of a table. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Relation (database)」の詳細全文を読む スポンサード リンク
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