Database management Systems edition 9 Chapter 3 Answers to 数据库管理系统9章3版的答案.docVIP

Database management Systems edition 9 Chapter 3 Answers to Review Questions 1. Define each of the following terms: Entity type A collection of entities that share common properties or characteristics. Entity-relationship model A logical representation of the data for an organization or for a business area. Entity instance A single occurrence of an entity type. Attribute A property or characteristic of an entity type that is of interest to the organization. Relationship type A meaningful association between (or among) entity types. Identifier An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type. Multivalued attribute An attribute that may take on more than one value for a given entity instance. Associative entity An entity type that associates the instances of one or more entity types and contains attributes that are peculiar to the relationship between those entity instances. Cardinality constraint Specifies the number of instances of one entity that can (or must) be associated with each instance of another entity. Weak entity An entity type whose existence depends on some other entity type. Identifying relationship The relationship between a weak entity type and its owner. Derived attribute An attribute whose values can be calculated from related attribute values. Multivalued attribute See letter g. Business rule A statement that defines or constrains some aspect of the business. Match the following terms and definitions: i composite attribute d associative entity b unary relationship j weak entity h attribute m entity e relationship type c cardinality constraint g degree a identifier f entity type k ternary l bill-of-materials 3. Contrast the following terms: Stored attribute; derived attribute A stored attribute is one whose values are stored in the database, while a derived attribute is one whose values can be calculated or derived from related stored attribute