Database. - Modeling of systems


The database is a key concept of database technology and a pivot control object in database systems. The definition of a database as a shared information resource of computerized technologies requires the specification of the notions data and information. Sometimes a database is treated as a "similarity of an electronic file", "storage for a set of data files stored in a computer" , meaning by the term "abstract file", not necessarily a physical disk file. " Obviously, with this view, data and information are considered synonymous. As a consequence, the assertion that in this case any data extracted in any way from the database is true becomes the information.

The classic definition of "database is data and relationships between them". seems to be more accurate and appropriate, taking into account the above considerations. Then the data extracted from the database on the basis of established links is information. Otherwise, the data extracted from the database requires interpretation. Of course, the data fragments stored in the database also correspond to the notion of information. Outside the links, the data is information only if they are typed, or classified, and the classification scheme used is known. Taking into account the application of the relational approach, the links between the data can be divided into links of consistency (compatibility of attributive values ​​of the table definition of the application object) and the correspondence of the correspondence (compatibility of attributive values ​​of inter-table definition of the application object).

Thus, in the future, by the term database we mean a set of related data , on the one hand, being information, and on the other hand, which form the basis for obtaining information, as arbitrary combinations

Fig. 5.6. Full technological scheme of the implementation of PM

stored related data. Then database data and information are by definition synonymous.

Methods and tools for determining and manipulating the database. In the database technology, two main groups of mechanisms for determining and manipulating the database are defined.

The first group includes a set of methods and means for determining related data, including a formal description of data structures, as well as database administration. Methods and means of data definition realize some degree of informativeness stored in the database, depending on the capabilities and limitations of the accepted data model. Data definition is performed statically, because informative links between data are stored and entered into the database along with the actual data. At the initial stages of the development of database technology, it was the development of a powerful data definition language (JOD) that was the main direction of development. The long-term activity of the working group CODASYL [2] on the development of a developed IODE is well known. However, it was not possible to derive the data definition languages ​​to the level of common programming languages ​​for a number of reasons [3].

The second group consists of methods and means of data manipulation, which implement informative data binding in dynamics, in the process of access to the database. In the initial stages, the data manipulation languages ​​(JMDs) were reduced to the definition of a simple C ^ LL interface, but at the turn of the 1980s the trend of development of the NMD practically closed the direction of the development of the IODR. Thanks to the wide application of the relational model, the manipulation languages ​​have been able to pass the way of becoming to the level of common programming languages. The most famous representative of the YMD family today is the language SQL (Structured Query Language) [2], which forms the basis and is itself the international standard of NMD.

Many users often get the impression that regardless of the type of the designed database using the SQL language, you can access any information based on the data stored in the database. In fact, this is far from the case. The SQL language does provide random access to database tables in any combination and combination. But getting information from the database is limited by the possibilities of communication between the data stored in the database. In the absence of these relationships, it is common practice to embed the processing of the links between the data in the application code. Then we can distinguish two main components of database manipulation:

- the actual manipulation language as the tool;

- procedures of data binding and management of information retrieval from the database, implemented by means of NMD.

For the relational approach, the most common procedural way to manage the retrieval of information from the database. There are three main methods of implementing this method:

1) Data binding and data manipulation modules are built into applications by programming in professional environments (MS Visual Studio, C + + Builder, Dlphx)

2) the binding and manipulation modules are developed in SgL-server languages ​​and stored directly in the server database, becoming also shared information resources

3) the binding and manipulation modules are designed in the form of system dynamic DLLs , thus creating access to the database in the form of a system fVindows - resource.

The construction of modules for linking and manipulating the database in the form of shared information resources in the SQL - environment of the servers or in the video of the system DLLs greatly approximates the aggregate content of such databases to the classical definition. It is characteristic that the realization of the database obtained in this way according to the complete technological scheme of Fig. 5.6 remains within the boundaries of the relational approach.

Varieties of database systems. Depending on how you define and manipulate the related data of the system, the database can be divided into the following main varieties.

Systems with file databases as databases use simple structured files in the formats bc1 , etc., and all informative links are defined and processed in applications that use such databases. The effectiveness of organizing structured files is usually increased by building indexes and other pointing systems, which, generally speaking, is typical for the creation of files. Indexes, as a rule, key fields of structures with the purpose of speeding up access (by sorting indexes), ensuring the uniqueness of field values, prohibiting the existence of undefined values, etc. One of the most significant drawbacks of file database systems (only in terms of their use) to attribute complete dependence on applications. Access to the information of file databases is possible only through a software-containing application. Obviously, as a shared information resource, file databases can exist only in symbiosis with data-binding applications. Software implementation of connections on servers or in the form of db -class libraries naturally gives the file DBs a completely new quality of a really shared information resource.

The opposite type includes such systems of the database in which all relationships between data are defined as data and stored in the database. Such systems can be called systems with object databases. The essence of the object database name is to achieve complete independence of object databases from applications. Object databases are full-fledged, independent resources of computerized technologies, which constitutes the main advantage of their application. At the same time, a complete definition of all the relationships between the data generates a significant complexity in the design of such databases.

Intermediate options for organizing databases, in which links are distributed between applications and databases, determine the variety of systems with application databases. The essence of the name reflects a weak or strongly pronounced orientation of the organization of the application database on the needs of applications that use it. Obviously, as a compromise option, applied databases can act as an optimized information resource of computerized technologies. Applied databases are widely used in computer simulation of specific systems 3.

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