数据库系统概念 第一章 习题答案

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1.7 List four applications you have used that most likely employed a database system to store persistent data.
Banking: For account information, transfer of funds, banking transactions.
• Universities: For student information, online assignment submissions, course registrations, and grades.
• Airlines: For reservation of tickets, and schedule information.
• Online news sites: For updating new, maintainence of archives.
• Online-trade: For product data, availability and pricing informations, order-tracking facilities, and generating recommendation lists.

1.8 List four significant differences between a file-processing system and a DBMS.
（1）这两个系统都包含一个数据集合和一组访问该数据的程序。数据库管理系统同时协调对数据的物理和逻辑访问，而文件处理系统只协调物理访问。
（2）数据库管理系统通过确保物理数据适合所有被授权访问它的程序来减少重复数据量，而文件处理系统中由一个程序写入的数据可能不会被另一个程序读取。
（3）数据库管理系统设计为允许灵活访问数据（即查询），而文件处理系统设计为允许对数据的预定义访问（即编译程序）。
（4）一个数据库管理系统被设计用于协调多个用户同时访问同一数据。文件处理系统通常被设计为允许一个或多个程序同时访问不同的数据文件。在文件处理系统中，只有当两个程序都可以只读访问该文件时，两个程序才能同时访问一个文件。

1.9 Explain the concept of physical data independence, and its importance in database systems.
物理数据独立性是指应用程序不依赖于物理模式。物理数据独立性使物理模式可以在应用程序丝毫不受影响的情况下被修改。

1.10 List five responsibilities of a database-management system. For each responsibility, explain the problems that would arise if the responsibility were not discharged.
数据库管理系统的五个功能包括：
1)与文件管理系统交互
2)完整性的强制执行
3)安全性强制执行
4)备份和恢复
5)并发性控制
若给定的数据库管理系统DMS没有实现以上功能，相应会有如下结果：
1）无法检索文件中存储的任何内容
2）一致性限制可能不被满足
3）未经授权的用户可以访问数据库，或者被授权访问数据库部分的用户可以访问数据库中他们缺乏权限的部分。
4）数据可能会永久丢失，而不是至少在故障之前存在的一致状态下可用。
5）尽管在每个事务中执行适当的完整性，但可能违反一致性约束。

1.11 List at least two reasons why database systems support data manipulation using a declarative query language such as SQL, instead of just providing a library of C or C++ functions to carry out data manipulation.
1）声明式语言对程序员来说更容易学习和使用。
2）程序员无需担心如何高效查询，具体技术由数据库完成。声明性规范使数据库系统更容易地做出正确的执行技术选择。

1.12 Explain what problems are caused by the design of the table in Figure 1.4.
Answer:
• If a department has more than one instructor, the building name and budget get repeated multiple times. Updates to the building name and budget may get performed on some of the copies but not others, resulting in an inconsistent state where it is not clear what is the actual building name and budget of a department.
• A department needs to have at least one instructor in order for building and budget information to be included in the table. Nulls can be used when there is no instructor, but null values are rather difficult to handle.
• If all instructors in a department are deleted, the building and budget information are also lost. Ideally, we would like to have the department information in the database irrespective of whether the department has an associated instructor or not, without resorting to null values.

1.13 What are five main functions of a database administrator?
模式定义；存储结构及存取方法定义；模式及物理组织的修改；数据访问授权；日常维护

1.14 Explain the difference between two-tier and three-tier architectures. Which is better suited for Web applications? Why?
在两层应用程序体系结构中，应用程序在客户端计算机上运行，并直接与服务器上运行的数据库系统进行通信。相反，在三层体系结构中，客户端计算机上运行的应用程序代码会与服务器上的应用程序服务器通信，并且从不直接与数据库通信。三层结构更适合Web应用程序。

1.15 Describe at least 3 tables that might be used to store information in a social networking system such as Facebook.
Answer: Some possible tables are:
a. A users table containing users, with attributes such as account name, real name, age, gender, location, and other profile information.
b. A content table containing user provided content, such as text and images, associated with the user who uploaded the content.
c. A friendstable recording for each user which other users are connected to that user. The kind of connection may also be recorded in this table.
d. A permissions table, recording which category of friends are allowed to view which content uploaded by a user. For example, a user may share some photos with family but not with all friends.

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