数据设计外文翻译.docx

附录adata design 1. data types and value setsanyone who has ever programmed a computer is accustomed to the fact that you usually have to tell the computer whether the information stored in a particular location is an integer, a real number, a character string, a boolean va1ue, or whatever. most computers have one electronic circuit for adding two integers and another for adding two real numbers, and, of course, addition does not make any sense at all, in general, for character strings. because the pattern of bits in at storage location can usual1y be interpreted as any of these, the computer must be told what is intended. this specification is known as the type of the data. identifying a data object as being of a particular type has the effect of defining an internal representation for the object. it also specifies which operations may be performed on that object and their effects. for example, the internal representation of an integer in most modem mainframe computers is a string of 32 bits in 2s complement format. the external representation is a character string in which only the digits 0 to 9 and the minus sign may appear. suitable interpretations of all the standard arithmetic and logical operations are also implied when a data object is declared to be of type integer.it is also true (but perhaps less obvious) that declaring a data object to be an integer defines a collating system for occurrences of that object. that is, it specifies how the relatively complex operation, sort, should operate. specific algorithms for converting between interna1 and external representations are also implied.2. keysall attributes describe some aspect of an entity. some attributes perform the additional role of distinguishing one particular entity occurrence from all others of the same type. for example, if care is taken not to assign any employee number to more than one person, then knowing an individuals employee number should be sufficient to locate all of the other attribute values for that individual. an attribute that can be guaranteed to have a unique value for each entity is called a key.sometimes there may be more than one attribute of an entity that uniquely identifies each occurrence. for example, if the personnel records include social security numbers (and if it is felt that the controls on issuing duplicate social security numbers are adequate), then either employee number or social security number could be used as the key for identifying employees. keys are fundamental to virtually all methods of physically managing data in computer systems. when there is more than one possible key, the alternatives are referred to as candidate keys. normally, one of these will be select as the primary key and used to determine where the record will be stored. other candidate keys that may be used to facilitate retrieval are called secondary keys.3. database schemathis is a description of the data which is stored in the database and specifies what data elements are store and what access paths are provided between these elements. the database schema also contains specifications of privacy as well as integrity constraints. it is somewhat similar to the conceptual schema, but is a description of data rather than of reality. some aspects of reality which are described by data in the database schema. note, however, that the database schema does not specify how the data is actually stored or how access paths are provided. (ideally it should not refer to files, records, sets or the like.) it is, therefore, an implementation independent description and for this reason is sometimes referred to as logical schema.that part of the database schema which is of interest to a particular end-user or group of end users is called a database sub-schema. ideally, a sub-schema should be specified using a notation which is most appropriate for the use to which it is being put. database sub-schemas have several use : (1) they can be referred to by application programmers to see what access paths are available in that part of the database in which they are interested(2) similarly, they can be referred to by the end-users when they are using a report program generator or formulating queries etc. using a query language provided.(3) they can be used to divide the database into units for the specification of privacy constrains. for example, a sub-schema could be defined for which all users have read access but only one specified user has write (update) access.4. physical schemathe physical schema is a description of the physical structure of the database. if, for example, conventional indexed-sequential files are used to store the database, then this will be stated in the physical schema. it will also contain details of record formats, blocking factors etc. the physical schema is constructed as an essential part of the design process. however, it may also be used as an integeral part of he operational database system as discussed below. the physical schema is sometimes called the internal schema.5. physical storage structurethe structure in which the database actually resides is termed the physical storage structure. it typically consists of disc files, tapes, mainstore, indices and programs to manipulate these components.6. back-up and recovery systemthe back-up and recovery system is the module which rebuilds the database after corruption due to hardware or software failure.understanding the application tasksone of the often-neglected steps on building software is really understanding the end users job-the that computer automation is intended to support.occasionally, this is because the application itself is quite specialized; more often, it is because the approach to design tends to be data-oriented. frequently, these are the major question asked in the analysis:(1) what data should be captured(2) how should the data be processed(3) how should the data be reportedthese question expand into a series of sub question ,and include issues such as input forms, codes, screen layouts, computations, postings, corrections, audit trails, retention, storage volumes, processing cycles, report formatting, distribution, and maintenance. there are all vitally important areas. one difficulty, however, is that they all focus solely on data. people use data, but they do tasks. one might argue that while this may be true of professional workers. key-entry clerks really only transfer data from an input form to a keyboard; their tasks are very data-oriented. this is a fair portrayal of their jobs today, but is this a consequence of the real job that need to get done, or is it a symptom of the computer application? using humans as input devices, particularly for data that is voluminous, consistent in format (as on forms), and in a limited range of variability, is an expensive and antiquated, not to mention dehumanizing, method of capturing data.this may sound like so much philosophy, but it has practical import in the way application design is done. people use data, but they do tasks. and they dont do tasks through o completion one at a time. they do several tasks that are subset of or in intersection with each other, and they so them all at once, in parallel.when designers allow this idea to direct the analysis and creation of an application, rather than focusing on the data orientation that has been historically dominant, the very nature of the effort changes significantly. why have windowing environments been so successful.because they allow a user to shut down and exit one in order to begin another. the windowing environment comes closer to mapping the way people really think and work than the old one thing at a time approach ever did. this lesson should not be lost. it should be built upon.understanding the application tasks means going far beyond identifying the data elements, normalizing them, and creating screens, processing programs, and reports. it means really understanding what the users do and what their tasks are, and designing the application to be responsive to those tasks, not just to capture the data, associated with them. in fact, when the orientation is toward the data, the resulting design will inevitably distort the users tasks rather than support them.how do you design an application that is responsive to tasks rather than data.the biggest hurdle is simply understanding that focusing on tasks is necessary. this allows you to approach the analysis of the business from a fresh perspective.附录b数据设计1. 数据类型和值域任何一个曾经编过程的人都习惯了这样一个事实：你不得不告诉计算机信息是一个整数，实数，字符串，布尔值或其它的值。大多数计算机由一个整数加法电路，又有一个实数加法电路，而且，通常字符串相加当然没有任何意义。因为在内存中的比特资料可以被解释成任何一种形式，计算机必须被告诉去做什么。这就是我们熟知的类型。定义资料对象为一个特定的类型对事物内部代表的定义有影响。举例来说，在大多数现代计算机框架内部整数表示为32位比特数的形式。在外部，整数表现为从0到9的数字符串的形式，可能带有负号。当一个资料对象被定义为整数类型时，所有标准的算术和逻辑运算都要做合适的解释。在定义一个数据类型为整形时，定义它的整理系统的事件也同样如此（可能不是如此明显）。那就是说，要定义相对复杂的操作，如排序，是如何进行的。内部表示和外部表示的转换算法也是必须的。2. 关键词所有的属性都描述实体的某一方面。一些属性扮演附加的角色：从所有其它同一类型的实体事例中区分某一特定实例。举个例子，如果我们注意不用一个雇员后标记一个以上的雇员，我们可知一个雇员的雇员号足以固定雇员的其它属性值。能确保每个实体都有唯一值的属性叫做关键词。有时可能不只一个属性唯一的确定每个实体。举个例子，如果人事纪录包含社会安全号（而且认为足以控制社会安全号的重复），那幺雇员号和社会安全号都可作为确定雇员的关键词。关键词在计算机系统中管理资料的所有物理有效方法中是最基本的。当有多余一个的可用关键词时，可供选者的为候选关键词。通常，我们选出一个作为主关键词，并决定纪录的位置。其它便于检索的候选关键词被称为副关键词。3. 数据库模式用于描述存储数据库中的数据和明确记载数据中的数据包括什么元素，即这些元素通过怎样的访问路径被访问。它也包含了安全设置出完整限制之外，与概念模式由几分相似。但对数据的描述将更加真实，一些真实反映由数据库中的数据所描述，笔记上，并不能明确记载数据库中的数据是如何被存储及访问路径如何被提供。（理想的它是不会参考文件、记录、设置或其他同类的东西）它是，因此独立描述及为这结论作为逻辑模式作为参考。最终用户或最终用户组对数据库中的某部分模式特别感兴趣被称为数据库代替-模式，理想的，代替-模式将利用字符明确记载数据存放位置与最适合的位置，数据库代替-模式有以下几种功能：（1）它们能利用应用程序，使它们所感兴趣的内容被访问的路径可见。（2）同样的，它们可以在最终用户的使用报告程序发电机和使用一种查询语言等等被体现。（3）它们能被用于划分数据单位为按去劝秘密的说明书。例如，代替模式中规定，每类级别上的用户可进行那些操作，并不仅指定少数用户有修改更新的权利。4. 物理模式物理模式是以物理的结构描述数据库，如果，例如传统的编入索引的-顺