Relation Schema and Instance A1,A2,...,An are attributes R=(A1,A2,...,An)is a relation schema Example: instructor =(ID,name,dept_name,salary) Formally,given sets D1,D2,...Dn a relation r is a subset of D X D2 X ..X Dn Thus,a relation is a set of n-tuples(a,a2,...,a)where each a;e D The current values(relation instance)of a relation are specified by a table An element t of r is a tuple,represented by a row in a table Database System Concepts-6th Edition 2.4 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 2.4 ©Silberschatz, Korth and Sudarshan th Edition Relation Schema and Instance A1 , A2 , …, An are attributes R = (A1 , A2 , …, An ) is a relation schema Example: instructor = (ID, name, dept_name, salary) Formally, given sets D1 , D2 , …. Dn a relation r is a subset of D1 x D2 x … x Dn Thus, a relation is a set of n-tuples (a1 , a2 , …, an ) where each ai  Di The current values (relation instance) of a relation are specified by a table An element t of r is a tuple, represented by a row in a table