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本课件对应4.1 Introduction to DBMS（数据库管理系统概述）一节，说明数据库管理系统的组成与功能，比较文件系统与数据库系统在数据共享与并发控制上的差别。课文配中英文对照译文、生词短语表与课后练习，可用于课堂讲授与课前预习。
| 单词 | 词性 | 中文释义 |
|---|---|---|
| compiler | n. | 编译器 |
| coordination | n. | 同等，调和 |
| concurrency | n. | 同时（或同地）发生，同时存在，合作 |
| clearance | n. | 清除 |
| distinguish | v. | 区别，辨别 |
| Duplication | n. | 副本，复制 |
| destruction | n. | 破坏，毁灭 |
| fundamental | adj. | 基础的，基本的 |
| 单词 | 词性 | 中文释义 |
|---|---|---|
| instantaneously | adv. | 瞬间地，即刻地，即时地 |
| legacy | n. | 遗赠（物），遗产 |
| navigate | v. | 导航，航行，航海，航空 |
| permanently | adv. | 永存地，不变地 |
| resiliency | n. | 跳回，弹性 |
| reservation | n. | 保留，（旅馆房间等）预定，预约 |
| transaction | n. | 办理，学报，交易，处理事务 |
| unauthorized | adj. | 未被授权的，未经认可的 |
| 短语 | 中文释义 |
|---|---|
| at the exact instant | 此时，与此同时，正当此时 |
| associate with | 联合，结合 |
| at will | 随意，任意 |
| high-level languages | 高级语言 |
| take into account | 重视，考虑Abbreviations |
A database management system (DBMS)is an important type of programming system, used today on the biggest and the smallest computers.[1]As for other major forms of system software, such as compilers and operating systems, awell-understood set of principles for database management systems have been developed over the years, and these concepts are useful both for understanding how to use these systems effectively and for designing and implementing DBMS's.
语言点分析：并列句。句首的Asfor用来转换话题，起到补充前文又突出后文的作用，可译为：至于。例句：As for the future of garments,I think fashion will become garish.至于未来的衣服，我认为时尚会变得过分装饰。句中的suchas…systems之间的部分是插入语，翻译时可以把两个逗号作为括号翻译出来。
DBMS简介
数据库管理系统是编程系统中的重要一种，现今可以用在最大的以及最小的电脑上。至于其他主要形式的系统软件（如编译器以及操作系统），近些年来开发出一系列容易理解的数据库管理系统规则，并且这些概念既有助于理解如何有效利用系统，又可以帮助设计和实现DBMS系统。
DBMSisa collection of programs that enables you to store, modify, and extract information from a database. There are many different types of DBMS's, ranging from small systems that run on personal computers to huge systems that run on mainframes.
DBMS是一个程序的集合，它使你能够存储、修改以及从数据库中提取信息。有很多不同类型的DBMS系统，从运行在个人电脑上的小型系统到运行在大型主机上的巨型系统。
DBMSqualifie s Therearetwo qualities that distinguish database management systems from other sorts of programming systems. 1)The ability to manage persistent data,and2)The ability to access large amounts of data efficiently.有两种功能使数据库管理系统区别于其他程序设计系统：1)管理固有数据的能力，以及2)高效访问大量数据的能力。
第一点只是表明现有一个固定存在的数据库；该数据库的内容是DBMS所要访问和管理的那些数据。第二点将DBMS和同样能管理固有数据的文件系统区分开来。在数据量非常大的时候最需要用到DBMS系统的功能，因为对于少量数据而言，简单的访问技术（如对数据的线性扫描）就足够了。
Point 1)merely states that there is a database that exists permanently; the contents of this database are the data that a DBMSaccesses and manages. Point 2)distinguishes a DBMSfroma filesystem, which also manages persistent data. ADBMS's capabilities are needed most when the amount of data is very large, because for small amounts of data, simple access techniques, such as linear scans of the data, are usually adequate.
[2]While we regard the above two properties of a DBMSas fundamental, there area number of other capabilities that are almost universally found in commercial DBMS's. Theyare:
Support for atleast one data model, or mathematical abstraction through which the user can view the data.
Support for certain high-level languages that allow the user to define the structure of data, access data, and manipulate data.
语言点分析：复合句，while放于句首引导让步状语从句，翻译为“虽然”，后半句转折，英文中不用“but”，但在翻译时需要翻译出“虽然……，但是……”。
虽然我们将以上两点作为DBMS的基本特性，但是其他一些功能在商业DBMS系统中也是常见的，它们是：
支持至少一种用户可以据之浏览数据的数据模式或数学提取方式。
支持某种允许用户用来定义数据的结构、访问和操纵数据的高级语言。
Transaction management, the capability to provide correct, concurrent access to the database by many users at once.
Access control, the ability to limit access to data by unauthorized users, and the ability to check the validity of data.
Resiliency, the ability to recover from system failures without losing data.
事务管理，即对多个用户提供正确地同时访问数据库的能力。
访问控制，即限制未被授权用户访问数据以及检测数据有效性的能力。
恢复功能，即能够从系统错误中恢复而不丢失数据的能力。
数据模型每个DBMS提供了至少一种抽象数据模型，该模型允许用户以更容易理解的术语而不是以原始比特位的方式察看信息。实际上，通常可以做到以几个不同抽象级别观察数据。在相对低的级别中，DBMS一般允许我们将数据形象化为由文件组成。
Data Models Each DBMSprovides atleast one abstract model of data that allows the user to see information not asrawbits, but in more understandable terms. Infact, it is usually possible to see data at several levels of abstraction. At a relatively low level, a DBMScommonly allows us to visualize data as composed of files.
Efficient File Access The ability to store a file is not remarkable: the filesystem associated with any operating system does that. The capability of a DBMSisseen when we access the data of a file. Forexample, suppose we wish to find the manager of employee"Clark Kent". If the company has thousands of employees, Itisvery expensive to search the entire file to find the one with NAME="Clark Kent".
高效数据访问存储一个文件的能力并不特别：操作系统中结合的文件系统都能做到。DBMS的能力在我们访问文件的数据时才能显示出来。比如，假设我们希望找到员工经理“克拉克·肯特”。如果这个公司有数千员工，那么要通过NAME＝“克拉克·肯特”搜索整个文件来找到这个人是非常费时的。
ADBMShelpsus to setup"index files,"or"indices,"that allow us to access the record for"Clark Kent"in essentially one stroke no matter how large the file is. Likewise, insertion of new records or deletion of old ones can be accomplished in time that is small and essentially constant, independent of the file’s length. Another thing a DBMShelpsus do is to navigate among files, thatis, to combine values in two or more files to obtain the information we want.
而DBMS帮助我们建立“索引文件”或“索引”，不管文件有多大，它都使我们能够一举访问到“克拉克·肯特”的记录。同样的，不管文件大小，新记录的插入或者原有记录的删除都可以在较短并且基本稳定的时间内完成。DBMS还可以帮助我们进行文件间的导航，即，通过结合两个或更多文件的值来获得我们所需的信息。
Query Languages Tomakeaccess to files easier, a DBMSprovides a query language, or data manipulation language, to express operations on files. Query languages differ in the level of detail they require of the user, with systems based on the relational data model generally requiring less detail than languages based on other models.
查询语言为了使访问文件更容易，DBMS提供了查询语言（或者说数据控制语言）表达对文件的操作。查询语言对用户所提供的细节的详细程度要求有所不同，基于关系数据模型的系统通常比基于其他模型的语言所需的细节要少。
Transaction Management Another important capability of a DBMSisthe ability to manage simultaneously large numbers of transactions, which are procedures operating on the database. Some databases are so large that they can only be useful if they are operated upon simultaneously by many computers:
事务管理DBMS的另外一项重要功能就是同时管理大量事务的能力，事务即数据库中运行的进程。某些数据库是如此之大，它们只有同时运行于多台计算机上才能发挥作用：
often these computers are dispersed around the country or the world. The database systems used by banks, accessed almost instantaneously by hundreds or thousands of automated teller machines (ATM), as well as by an equal or greater number of employees in the bank branches, is typical of this sort of database. Anairline reservation system is another good example.
通常这些计算机分散在全国甚至世界各地。银行中使用的数据库系统就是这类数据库的一个典型，它们几乎同时被成千上万的自动取款机所访问，也同时被同样多甚至更多的支行员工所访问。机票预定系统是另一个好例子。
Sometimes, two accesses do not interfere with eachother. Forexample, any number of transactions can be reading your bank balance at the same time, without any inconsistency.[3]Butifyouare in the bank depositing your salary check at the exact instant your spouse is extracting money from an automatic teller, the result of the two transactions occurring simultaneously and without coordination is unpredictable.
语言点分析：这是一个复合句，主句为“theresultof…unpredictable”，前面部分为if所引导的表假设的条件从句。条件从句中的your spouse is extracting money from an automatic teller是定语从句，修饰theexactinstant。
有时两个访问不会互相干扰。例如，任意多的事务可以同时读取你银行的结余而不引起任何冲突。但是如果你正在银行里办理工资存款，与此同时，你的配偶在一台自动取款机上取款，两个事务同时发生且没有彼此协调，那你的查询结果就很难说了。
Thus, transactions that modify a data item must“lockout”other transactions trying toreador write that item at the same time. ADBMSmust therefore provide some form of concurrency control to prevent uncoordinated access to the same data item by more than one transaction.
因此，会引起数据项改变的事务必须“上锁”，将其他在同一时刻试图读写该项数据的事务关在外面。因此，DBMS必须提供某种并发控制状态以阻止多个事务对于同一数据项的非协调访问。
更复杂的问题发生在数据库分布在许多不同计算机系统上的时候，它们可能使用数据副本来允许高速的本地访问以及避免由于某台计算机崩溃而破坏数据。
Even more complex problems occur when the database is distributed over many different computer systems, perhaps with duplication of data to allow both faster local access and to protect against the destruction of data if one computer crashes.
Security of Data ADBMSmustnot only protect against loss of data when crashes occur, as we just mentioned, but it must prevent unauthorized access. Forexample, only users with a certain clearance should have access to the salary field of an employee file.
数据安全如上文提到的那样，DBMS不仅可以在计算机崩溃时保护数据不丢失，而且还能够阻止非法访问。比如，只有拥有特定权限的用户可以访问职工文件的工资区域。
DBMSTypes
Designers developed three different types of database structures: hierarchical, network, and relational. Hierarchical and network were first developed but relational has become dominant. While the relational design is dominant, the older databases have not been dropped.
DBMS类型
设计人员开发了三种不同类型的数据库结构：层次数据库、网状数据库以及关系数据库。层次数据库和网状数据库是首先被开发出来的，但关系数据库已经成为了主流数据模型。尽管关系数据库的设计已经成为主流，但旧的数据库仍然没有被遗弃。
Companies that installed a hierarchical system such as IMSinthe 1970 s will be using and maintaining these databases for years to come even though new development is being done on relational systems. These older systems are often referred to as legacy systems.
尽管关系数据库不断得到发展，但在20世纪70年代安装了层次数据库的一些公司，如IMS，在未来几年仍然将维持使用这些数据库。这些旧的数据库系统通常被称作遗留系统。
习题4.1 一 判断正误：根据课文内容判断下列说法正误。
Put“true”or“false”in the brackets for the following statements according to the passage.
习题4.1 二 练习：根据课文内容完成下列练习。
Fillinthe blanks according to the passage.
习题4.1 三 翻译：将下列单词和短语译成中文。
Translate the following words and expressions into Chinese.
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