Đang chuẩn bị liên kết để tải về tài liệu:
Ebook Fundamentals of database systems (6th edition): Part 2

(BQ) Part 2 book "Fundamentals of database systems" has contents: Database design theory and normalization; file structures, indexing, and hashing; query processing and optimization, and database tuning; transaction processing, concurrency control, and recovery;.and other contents. | part 6 Database Design Theory and Normalization This page intentionally left blank chapter 15 Basics of Functional Dependencies and Normalization for Relational Databases I n Chapters 3 through 6, we presented various aspects of the relational model and the languages associated with it. Each relation schema consists of a number of attributes, and the relational database schema consists of a number of relation schemas. So far, we have assumed that attributes are grouped to form a relation schema by using the common sense of the database designer or by mapping a database schema design from a conceptual data model such as the ER or Enhanced-ER (EER) data model. These models make the designer identify entity types and relationship types and their respective attributes, which leads to a natural and logical grouping of the attributes into relations when the mapping procedures discussed in Chapter 9 are followed. However, we still need some formal way of analyzing why one grouping of attributes into a relation schema may be better than another. While discussing database design in Chapters 7 through 10, we did not develop any measure of appropriateness or goodness to measure the quality of the design, other than the intuition of the designer. In this chapter we discuss some of the theory that has been developed with the goal of evaluating relational schemas for design quality—that is, to measure formally why one set of groupings of attributes into relation schemas is better than another. There are two levels at which we can discuss the goodness of relation schemas. The first is the logical (or conceptual) level—how users interpret the relation schemas and the meaning of their attributes. Having good relation schemas at this level enables users to understand clearly the meaning of the data in the relations, and hence to formulate their queries correctly. The second is the implementation (or 501 502 Chapter 15 Basics of Functional Dependencies and Normalization