【正文】 in general, for character strings. Because the pattern of bits in at storage location can usual1y be interpreted as any of these, the puter 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 puters is a string of 32 bits in 239。s plement 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 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 plex operation, sort, should operate. Specific algorithms for converting between interna1 and external representations are also implied.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 individual39。s 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 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 puter 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.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 enduser or group of end –users is called a database subschema. Ideally, a subschema should be specified using a notation which is most appropriate for the use to which it is being put. Database subschemas have several use : (ⅰ) 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, (ⅱ) similarly, they can be referred to by the endusers when they are using a report program generator or formulating queries etc. using a query language provided (see enduser interface below) and (ⅲ) they can be used to divide the database into units for the specification of privacy constrains. For example, a subschema could be defined for which all users have read access but only one specified user has write (update) access.PHYSICAL SCHEMAThe physical schema is a description of the physical structure of the database. If, for example, conventional indexedsequential 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.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 ponents.BACKUP AND RECOVERY SYSTEMThe backup and recovery system is the module which rebuilds the database after corruption due to hardware or software failure.Understanding the Application TasksOne of the oftenneglected steps on building software is really understanding the end user’s jobthe that puter 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 dataoriented. Frequently, these are the major question asked in the analysis:■ What data should be captured?■ How should the data be processed?■ How should the data be reported?These question expand into a series of sub question ,and include issues such as input forms, Codes, screen layouts, putations, 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. Keyentry clerks r