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1、Unit 6 Database,Text A: Enter the World of Relational Database Text B: Database Management Systems Grammar Focus,Text A: Enter the World of Relational Database,下一页,返回,Text A: Enter the World of Relational Database,In todays business world, information technology is in its full boom. Processing data
2、and pumping out information has become the focal point of the modern day computing environment. Data can be stored in various ways, depending on how it will be used later on. This data store is called a database. The rawest form of data storage is a file where data is stored without any attributes,
3、characteristics or links. It requires a lot of programming effort to process this type of data, so the relational data model evolved. The relational data model represents data and relationships among the data elements. This model is based on real world scenarios, which consist of basic objects known
4、 as entities and how these entities are connected with each other, which constitutes the relationships. A presentation of this concept is known as an ERD,上一页,下一页,返回,Text A: Enter the World of Relational Database,(entity relationship diagram). Lets take a simple example of a database for a bookstore.
5、 Book titles and authors are two entities in it (among many others). The relationship between authors and books is that many authors can write one book and one author can have several books. This type of relationship is called an “any to many” relationship. Schematic representations of these relatio
6、nships are shown in Figure 5-1.,上一页,下一页,返回,Text A: Enter the World of Relational Database,To identify all the entities and their relationships is the first step of relational data modeling. This is also known as “logical database design.” Once the logical design is completed, it requires creating so
7、me kinds of physical objects (known as “table”) to store this information. We should need a system that would facilitate storing entities with their attributes and relationships. This system should also provide the capability of retrieving the desired information about the entities.,上一页,下一页,返回,Text
8、A: Enter the World of Relational Database,. Introduction to a Database Management System A database management system is a computer based system to record interrelated data, and it provides interfaces to access the data. A database system is designed to handle a large amount of data and is both conv
9、enient and efficient to use. The overall purpose of a database system is to maintain information in such a way that: Data redundancy is reduced Data integrity is maintained Data access is easy Data is secured Data sharing is possible,上一页,下一页,返回,Text A: Enter the World of Relational Database,Words an
10、d Expression sanomaly: n. 异常（现象） associate: v. 联系，联合 attribute: n. 属性 characteristic: adj. 典型的，独特的 concurrent: adj. 同时发生，并发的 constitute: v. 组成，构成 diagram: n. 图示，图解 duplication: n. 重复，复制 entity: n. 实体 ,上一页,下一页,返回,Text A: Enter the World of Relational Database,facilitate: v. 使更容易 focal： adj. 焦点的 incon
11、sistency: n. 不一致 interface: n. 界面 interrelate: v. 使相互关联 rawest: adj. 最原始的 redundancy: n. 冗余 relational: adj. 有关系的，相关的 relationship: n. 联系 representation: n. 体现，表示 retrieve: v. 恢复，重获,上一页,下一页,返回,Text A: Enter the World of Relational Database,scenario: n. 方案，设想 pumping out: 提取，抽取 vice versa: 反之亦然 compu
12、terbased: 基于计算机的 due to: 因为，由于,上一页,下一页,返回,Text A: Enter the World of Relational Database,Notes This model is based on real world scenarios, which consist of basic objects known as entities how these entities are connected with each other, which constitutes the relationships. 这种模型基于现实世界的情况，包括被称为实体的基本
13、对象以及构成这些实体间关系的互联方式。 A database management system is a computerbased system to record interrelated data, and it provides interfaces to access the data. 数据库管理系统是记录相互关联数据的基于计算机的系统，它为访问数据提供界面。,上一页,下一页,返回,Text A: Enter the World of Relational Database,Data duplication is minimized as much as possible, wh
14、ich is an efficient storage strategy that eliminates data inconsistencies. 数据重复应尽可能地减少，这是消除数据不一致的有效存储策略。 The data inaccuracy can be due to inconsistencies (as discussed previously) or due to not following some valid constraints e.g., 30th day of February is an inaccurate data value. A DBMS can suppo
15、rt multiple users and not every user should be able to access all the data. 数据库管理系统支持多用户，但也不意味着每个用户可以接触数据库里的所有数据。,上一页,返回,Text B: Database Management Systems,The term database is often to describe a collection of related files that is organized into an integrated structure that provides different peo
16、ple varied access to the same data. In many cases this resource is located in different files in different departments throughout this organization, often known only to the individuals who work with their specific portion of the total information. In these cases, the potential value of the information goes unrealized because a person in other departments who may need it does not know it or it cann
