【数据库系统原理】关系模型.pptVIP

第三章：关系模型Chapter 3: Relational Model 关系数据库的结构Structure of Relational Databases 关系代数Relational Algebra 元组关系演算Tuple Relational Calculus 域关系演算Domain Relational Calculus 扩展关系代数操作Extended Relational-Algebra-Operations 数据库更新Modification of the Database 视图Views 关系示例Example of a Relation 基本结构Basic Structure 给定一组域集，则关系是该域集的笛卡尔积的子集。即关系是N元集合，其中每个元素都隶属于某个域。Formally, given sets D1, D2, …. Dn a relation r is a subset of D1 x D2 x … x DnThus a relation is a set of n-tuples (a1, a2, …, an) where ai ? Di 例：Example: if customer-name = {Jones, Smith, Curry, Lindsay} customer-street = {Main, North, Park} customer-city = {Harrison, Rye, Pittsfield}Then r = { (Jones, Main, Harrison),  (Smith, North, Rye), (Curry, North, Rye), (Lindsay, Park, Pittsfield)} 其中，r是基于上述三个域的关系。is a relation over customer-name x customer-street x customer-city 属性类型Attribute Types 每个关系的属性都有一个名称。Each attribute of a relation has a name 各属性的取值范围集称为属性的域。The set of allowed values for each attribute is called the domain of the attribute 属性的值通常要求是原子性的，即不能分割。Attribute values are (normally) required to be atomic, that is, indivisible 例如，多值属性可以不是原子性的。E.g. multivalued attribute values are not atomic 例如，组合属性值不是原子性的。E.g. composite attribute values are not atomic 空值这一特殊的值包含在所有的域中。The special value null is a member of every domain 空值导致了许多操作定义的复杂性。The null value causes complications in the definition of many operations 在主体表示中可以先忽略空值，将其放在以后再考虑。 we shall ignore the effect of null values in our main presentation and consider their effect later 关系模式Relation Schema A1, A2, …, An 是属性are attributes R = (A1, A2, …, An )是关系模式 is a relation schema 例如，E.g. Customer-schema = (customer-name, customer-street, customer-city) r(R) 是关系模式R中的一个关系。is a relation on the relation schema R 例如，E.g. customer (Customer-schema) 关系实例Relation Instance 关系的当前值（关系实例）用表来表示。The current values (relation instance) of a relation are specified by a table r中元素t称为无组，