1. Database function: A database is a warehouse that organizes, stores and manages data according to the data structure. It came into being more than 60 years ago. With the development of information technology and market, especially after 1990s, data management is no longer just storing and managing data, but changing into various data management methods that users need. There are many types of databases, from the simplest tables with various data to large-scale database systems that can store massive data, which have been widely used in all aspects.
In the information society, the full and effective management and utilization of various information resources is the premise of scientific research and decision-making management. Database technology is the core part of various information systems such as management information system, office automation system and decision support system, and it is an important technical means of scientific research and decision management.
2. Definition of database 1: A database is a warehouse built on computer storage equipment, which organizes, stores and manages data according to the data structure.
Simply put, it can be regarded as an electronic filing cabinet-a place to store electronic documents. Users can add, intercept, update and delete data in documents.
In the daily work of economic management, it is often necessary to put some relevant data into such a "warehouse" and handle it according to the needs of management.
For example, the personnel department of enterprises and institutions often stores the basic information of employees (job number, name, age, gender, place of origin, salary, resume, etc.). ) in a table, can be regarded as a database. With this "data warehouse", we can query the basic situation of an employee at any time as needed, and we can also query the number of employees whose wages are within a certain range, and so on. If all these tasks can be done automatically on the computer, then our personnel management can reach a very high level. In addition, in financial management, warehouse management and production management, it is necessary to establish many such "databases" in order to realize the automatic management of finance, warehouse and production by computer.
Three. Definition of Database 2: Strictly speaking, a database is an organized and pleasant collection of data stored in a computer for a long time. The data in the database is organized, described and stored in a certain data model, which has the characteristics of minimum redundancy, high data independence and easy expansion, and can be shared by multiple users in a certain range.
This kind of data set has the following characteristics: it is as non-repetitive as possible and serves a variety of applications of a specific organization in an optimal way. Its data structure is independent of the application program that uses it, and the addition, deletion, modification and retrieval of data are managed and controlled by unified software. From the development history, database is an advanced stage of data management, which was developed by file management system.
4. Database processing system: A database is a general data processing system in an organization or application field, which stores a collection of relevant data belonging to enterprises, institutions, groups and individuals. The data in the database is established from a global perspective, and is organized, described and stored according to a certain data model. Its structure is based on the natural relationship between data, so it can provide all necessary access paths. The data is no longer aimed at an application, but at the whole organization, and has the overall structural characteristics.
The data in the database is established for many users to enjoy their information, and has got rid of the restrictions and constraints of specific programs. Different users can use the data in the database according to their own purposes; Multiple users can enjoy the data resources in the database at the same time, that is, different users can access the same data in the database at the same time. The enjoyment of data not only meets the requirements of users for information content, but also meets the requirements of information exchange between users.
Verb (abbreviation of verb) Basic structure of database: The basic structure of database is divided into three levels, reflecting three different perspectives of observing database.
A database composed of internal patterns is called a physical database; The data composed of conceptual patterns is called conceptual database. A database composed of external patterns is called a user database.
(1) physical data layer.
It is the innermost layer of the database and a collection of data actually stored on the physical storage device. These data are original data, which are processed by users and consist of bit strings, characters and words processed by instruction operations described by internal modes.
⑵ Conceptual data layer.
It is the middle layer of the database and the overall logical representation of the database. It is pointed out that the logical definition of each data and the logical relationship between data are a collection of storage records. It involves the logical relationship of all objects in the database, not their physical conditions, and is a database under the concept of database administrator.
(3) User data layer.
It is a database that users see and use, representing a data set used by one or some specific users, that is, a logical record set.
The relationship between different levels of databases is transformed by mapping.
VI. Main features of the database: (1) data sharing.
Data sharing includes that all users can access the data in the database at the same time, and users can use the database in various ways through the interface and provide data sharing.
(2) Reduce data redundancy.
Compared with the file system, because the database realizes data sharing, users are prevented from creating application files separately. Reduce a large number of duplicate data, reduce data redundancy and maintain data consistency.
(3) independence of data
The independence of data includes logical independence (the logical structure of database and application is independent of each other) and physical independence (the change of physical structure of data does not affect the logical structure of data).
(4) Centralized data control.
In the file management mode, the data is in a decentralized state, and different users or the same user do nothing with their own files in different processes. Database can be used to centrally control and manage data, and data model can be used to express the organization of various data and the relationship between data.
5] data consistency and maintainability, to ensure the safety and reliability of data.
It mainly includes: ① security control: preventing data loss, wrong update and unauthorized use; ② Integrity control: ensure the correctness, validity and compatibility of data; ③ Concurrency control: Multiple accesses to data are allowed in the same time period, which can prevent abnormal interaction between users.
[6] Fault recovery
A set of methods provided by database management system can find and repair faults in time, thus preventing data from being destroyed. The database system can recover the faults occurred during the operation of the database system as soon as possible, which may be physical or logical errors. Such as data errors caused by system misoperation.
7. Database data types: Databases are usually divided into three types: hierarchical database, network database and relational database. Different databases are connected and organized according to different data structures.
1. data structure model
(1) data structure
The so-called data structure refers to the organizational form of data or the relationship between data.
If d represents data and r represents a set of relationships between data objects, then DS=(D, r) is called a data structure.
For example, there is a telephone directory in which the names and corresponding telephone numbers of N people are recorded. In order to find someone's phone number conveniently, names and numbers are arranged in dictionary order, and the corresponding phone number is followed by the name. In this way, if you want to find a person's phone number (assuming that the first letter of his name is Y), you only need to find those names that start with Y. In this example, the data set D is the name and phone number, and the relationship R between them is arranged in dictionary order, and its corresponding data structure is DS=(D, R), which is an array.
⑵ Data structure type
Data structure is divided into logical structure and physical structure of data.
The logical structure of data is to observe and analyze data from a logical point of view (that is, the connection and organization of data), regardless of the storage location of data; The physical structure of data refers to the structure of data stored in the computer, that is, the realization form of the logical structure of data in the computer, so the physical structure is also called storage structure.
Only the logical structure of data is studied here, and the method of reflecting and realizing data connection is called data model.
At present, there are three popular data models, namely hierarchical structure model and network structure model based on graph theory and relational structure model based on relational theory.
2. Hierarchical, mesh and relational database system
(1) hierarchy model
The hierarchical structure model is essentially a directed ordered tree with root nodes (mathematically, "tree" is defined as an acyclic connected graph). The picture below shows the organizational structure of an institution of higher learning. This organization chart is like a tree. School departments are roots (called root nodes), departments, majors, teachers and students are branches (called nodes), the connection between roots and branches is called edges, and the ratio of roots to edges is 1:N, that is, there is only one root and n branches.
The database system established according to hierarchical model is called hierarchical model database system. Ims (Information Management System) is its typical representative.
⑵ Network structure model
The database system established according to the mesh data structure is called mesh database system, and its typical representative is DBTG(Database Task Group). Grid data structure can be transformed into hierarchical data structure by mathematical methods.
⑶ relational structure model
Relational data structure simplifies some complex data structures into simple binary relations (that is, two-dimensional table form). For example, the employee relationship in a certain unit is a binary relationship.
A database system composed of relational data structures is called a relational database system.
In a relational database, almost all data operations are based on one or more relational tables, and data management is realized by classifying, merging, connecting or selecting these relational tables.
DBASE is a typical representative of this kind of database management system. For a practical application problem (such as personnel management), sometimes a variety of relationships are needed to achieve it. A relationship established with dBASE is called a database (or database file), and multiple databases established corresponding to multiple relationships are called database systems. Another important function of dBASE is to use and manage the database by creating command files. The command sequence file corresponding to the database system is called the database application system.
So simply speaking, a relationship is called a database, and several databases can form a database system. Database system can derive various types of auxiliary files and establish its application system.
? 8. Database types: network database, relational database, hierarchical database and object-oriented database. Commercial applications are mainly relational databases, such as Oracle, DB2, Sybase, MS SQL Server, Informax, MySQL, etc.