文档介绍：Dr. Byunggu Yu 1 RELATIONAL MODEL Schema and Relation Instance ????A relation schema is a list of attribute names (user defined domain names), . Account = (branch-name, account#, balance). However, for various reasons, a relation schema is often represented by a set (not an ordered list) of attributes. ????A relation instance is a subset of a Cartesian product of a list of domains. ., account(Account): "account" is a relation instance on "Account" schema. The relation instance "account" is a subset of D1 × D2 × D3, where D1, D2, and D3 are the domains of "branch-name", "account-name", and "balance", respectively. ????A tuple is an element of a relation instance. If there are n attributes, each tuple of the relation instance is an ordered n- tuple. Thus, each tuple is an n-ary relationship between n attribute values. A relation is, in fact, a set of such relationships. ????"relation" often means "relation instance" Dr. Byunggu Yu 2 ????"database" often means "database instance" ????Recall the three-level database (Lecture Note#1). In the logical level, database schema is a set of relation schemas and database instance is a set of relation instances. Note, in relational data model, relation schema consists of atomic domains. A dom