TAILIEUCHUNG - Beginning Database Design- P5

Beginning Database Design- P5:This book focuses on the relational database model from a beginning perspective. The title is, therefore, Beginning Database Design. A database is a repository for data. In other words, you can store lots of information in a database. A relational database is a special type of database using structures called tables. Tables are linked together using what are called relationships. You can build tables with relationships between those tables, not only to organize your data, but also to allow later retrieval of information from the database | Database Modeling Building Blocks Figure 3-15 shows a data diagram with authors on the right of the diagram and their respective publications on the left in a one-to-many relationship. One author has two titles another five titles another three titles two authors have one each and two other authors have nothing published at all at least not in this database. PUBLICATION_ID SUBJECT_ID AUTHOR_ID 1 7 2 2 7 2 3 7 3 4 7 3 5 7 3 6 7 3 7 7 3 9 7 4 10 7 4 11 7 4 8 15 6 12 16 7 - Cootfaii__ Ringworid ghe Complete Wor BocuS Pocus pities in Flight__ a Case of Conscience Foundation TITLE foundatio i and Em Foundation s Edge Prelude to Founda Lucifer s Hammer Second Foundation AUTHOR_ID . name 1 Orson Scott Card - 2 James Blish F 3 Isaac Azimov - 4 Larry Niven 5 Jerry Pournelle 6 William Shakespeare Kurt Vonnegut Figure 3-15 One-to-many implies one entry to many entries between two tables. Many-to-Many A many-to-many relationship means that for every one record in one table there are many possible records in another related table and visa versa for both tables . The classic example of a many-to-many relationship is many students enrolled in many courses at a university. The implication is that every student is registered for many courses and every course has many students registered. The result is a many-to-many relationship between students and courses. This is not a problem as it stands however if an application or end-user must find an individual course taken by an individual student a uniquely identifying table is required. Note that this new table is required only if unique items are needed by end-users or an application. In Figure 3-16 from left to right the many-to-many relationship between PUBLISHER and PUBLICATION tables is resolved into the EDITION table. A publisher can publish many publications and a single publication can be published by many publishers. Not only can a single publication be reprinted but other types of media such as an audio tape .

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