During the class we had been talking about:



3.3 Databases


A function of a computer system is, data, text, images, language, sounds store, so that their needs, particularly swift and easy retrieval and benefits possible.
For the user of a personal computer is often the situation on that information will be evaluated and processed, for example, in several different graphics, reports and spreadsheets are included. These documents are often using different software packages such as different file formats. The retrieval, and above all, the benefit of these documents therefore requires knowledge of each set PC file formats.
In this section, in particular, is a matter of how large volumes of data reliably and permanently stored and retrieved when needed them. Therefore it is necessary for one or more users relevant Ausschitte from reality isolate, and the latter in conventional file systems or databases way. Basic questions of the data organization and the planning of database systems and tools for the widespread use of databases are therefore more content this section.

3.3.1. From the file to the database

Files, and therefore those databases contain illustrations of the reality that for inter-operational and information and communication processes as necessary as, for example, prove useful. The completeness and accuracy, for example, a file or database can only involvement in the real environment assessed.

3.3.1.1. The objects file and sentence

A set is an object of simplistic reality in a computer system. One such object can be a concrete object, an event or an abstract concept. It is described by its attributes. The object "textbook" can be, for example, by the attributes "number", "Author", "title", "price", "Publishing" and "stock" to describe. The concrete contents of the specific attributes of a textbook will be assigned fields. All fields of a specific object constitute a record. A tabular presentation is suitable for excellent. The table can be to the description of other objects to be extended.
Data fields are the smallest unit of files and databases. Data fields are marked with a name, following often with the name of the attributes and can cover different date types. The fields "number" and "there" are like Type, "integer", the field "price" of the type "decimal number." The fields "author", "title" and "publishers" are of the type "text" and the "leaking" of the type "flag" (with the expression "y" or "n").

From similar deresn attribute objects and object classes are formed. A file is a great property; they consist of one or more date rates. In that field, or those areas in which a artefacts. Key are assembled from several fields, they are as combinations key.

Such structured files are sometimes referred to as flat, classic or conventional files. Bezuges because of the heavy tasks of services to meet individual user files in the conventional rule is not the inner - and inter-task diversity. A major lack of organisation means of data files is occurring redundancy and inconsistence within the file and other files za, the company will be led.


Characteristic of conventional files is also the strong, mutual dependence with the application accessing these programmes. Advances operational procedures, and legal or other changes often make adjustments in the structure of datein necessary. It must, for example, New fields added to a file. In the above example "lehrb <úcher" could be a change in the order - and lieferungs process (of the logistics) the file extension of the "book" to the field "isbn" require



3.3.1.2. Use of a conventional file

Based on the following examples are depicted in simple query calculation tables tools explained. Does the program will feature "special filter".
The process of a file from those records filters, which meet the specified criteria, the selection means. The basis of selection is the use of logical operators (and, or, not), comparison operators (equal, not equal, bigger, smaller, More immediately, small equal) arithmetic operators and special operators. If, however, from the present number of data fields only certain selected, it is called projection. Selection and projection can be executed in parallel.

3.3.1.3. DBMS database systems (management)

Database systems are characterized by the fact that by several / many users and applications used data in a global model (conceptual model).
There is a decoupling of the problems of data storage and data organization of the technical tasks of the users. Redundancies are only for reasons of security, integrity or faster access time tolerated. Inconsistencies will, as far as possible off.
Database systems are made up of the database and a DBMS. It should basically be whether the data model hierarchy, the network model or the most widely used, explained below relational model is based.



All that is the theory about we have been talking at class today and we have been learned with an example how to differenciate between specialfilter and standfilter in a database.
We have learned it by an example with books.
The special filters have not to be complex and have to be exposed to logic for the relationship to be meaningful and the computer will understand.
For example said: search chemistry books published by the company "X", that the computer understands it is only "A and B" for example.
Examples of criteria for filtering are "A and B" and "A or B" simple, not complex.All this is not what is known as "selection".
We have also seen that there is a database, the area where establishing criteria and the result area, all these fields we have seen directly on the computer with the example we have been doing in class, and thus the entire theory has been previously explained more clearly.