系统分析与设计课件 9.ppt

Chapter 9 Designing Databases Designing Database The process of database design Logical database design Four key steps 1.Develop a logic data model for each known user interface for the application using normalization principles. 2.Combine normalized data requirement from all user interfaces into one consolidated logical database model. 3.Translate the conceptual E-R data model for the application,developed without explicit consideration of specific user interfaces,into normalized data requirements. 4.Compare the consolidated logical database design with the translated E-R model and produce,through view integration,one final logical database model for the application. The process of database design Entities design Primary guidelines Choosing the storage fomat Grouping attributes from the logical database model into physical records Arranging related records in secondary memory so that they can be stored,retrieved,and updated rapidly. Selecting media and structures for storing data to make access more efficient. Primary key Its value is unique across all occurrences of a relation Relational Database Model Relational database model—data represented as a set of related table or relations. relation Two-dimensional table of data，consists of a set of named columns and an arbitrary number of unnamed rows. 屬性 每一列都是獨一的 欄位的順序可以互相交換 每一列也可互相交換 Well-Structured Relations A relation that contains a minimum amount of redundancy and allows users to insert,modify,and delete the rows without errors or inconsistencies. Normalization The process of converting complex data structures into simple,stable data structures. First normal form(1NF) 去除重覆群組 Second normal form(2NF) Each nonprimary key attribute is identified by the whole key(what we call full functional dependency) Third normal form(3NF) Nonprimary key attributes do not depend on each other(what we call no transitive dependencies ) The result of normalization is that every nonprimary key attribute depends upon the whole prim