《数据库原理与实用技术》第二章 关系模型The Relational Model.pdfVIP

Chapter 2 The Relational Model 有需要word版格式的可索取 The Relational Model  A database model, or data model, is a set of definition describing real-world information.  It also describes the types of operations available to access and update this information. 2.1 The CAP Database  The CAP database (for the table names CUSTOMERS, AGENTS, and PRODUCTS)  See Figure 2.1 and Figure 2.2 . 2.2 Naming the parts of a Database  Two different standard terminologies(术语) are used in databases. －One terminologies refers to tables, rows , and columns －Other refers to relations (corresponding to tables), tuples (元组，corresponding to rows), and attributes (corresponding to columns) 2.2 Naming the parts of a Database (1) A database is defined to be a set of named tables, or relations CAP={CUSTOMERS, AGENTS, PRODUCTS, ORDERS} (2) We define the heading of a table to be a set of columns whose names appear at the top of the table Head(CUSTOMES)={cid, cname, city, discnt} (3) The heading of a table is also referred to as a relational schema(关系模式), and the set of all relation schemas for a database is known as a database schema 2.3 Relational Rules RULE 1: First Normal Rule(第一范式规则). In defining tables, the relational model insists that : columns that have multi-valued attributes (sometimes called repeating fields) or have any internal structure (like a record) are NOT permitted! 2.3 Relational Rules RULE 1: First Normal Rule. In defining tables, Example: 2.3 Relational Rules RULE 1: First Normal Rule. In defining tables, Example: 2.3 Relational Rules RULE 1: First Normal Rule. In defining tables, Example: 2.3 Relational Rules RULE 2: Access Rows by Content Only Rule. Rule 2 states that : we can only retrieve rows by their content. 2.3 Relational Rules RULE 3: The Unique Ro