Question 1: What does database mean? Database

data base

In order to meet the needs of multiple users in a certain department, the interconnected data are organized, stored and used in the computer system according to a certain data model. data*** .

In addition to general hardware and software, a computer system with a database must have direct access storage devices to store large amounts of data, and software to manage and control the database - a database management system (DBMS) , the person who manages the database - database administrator (DBA). Such data, hardware, software and management personnel collectively constitute a database system. The database is only one component of the database system.

Functions and characteristics of database systems Database systems are developed from document systems. Compared with the document system, this system has three main characteristics: data, system and control.

Data characteristics In the file system, although access methods can be used to convert programs and data, files still correspond to applications, that is, data are still application-oriented. Each application creates its own set of documents. If different applications involve the same data, these data are included in their respective volumes. There is no connection between various records in the file, so the data redundancy is high. When adding new applications, new volumes must be added at the same time. Therefore, the documents in the document system are unstructured information that is not easy to expand. The database not only describes the data itself, but also describes the connections between the data. Its data structure reflects the overall information structure of a certain department. The data redundancy is small and it is easy to expand new applications. Therefore, it is an information library oriented to the overall data structure and can be shared by multiple users.

System Characteristics All data has both logical and physical aspects. In a database system, the description of the logical structure of data is called a logical schema. Logical patterns are further divided into global patterns (referred to as patterns) that describe the global logical structure and sub-patterns that describe the local logical structures involved in certain applications. The description of the physical structure of data is called a storage schema. These two modes are collectively called database modes.

In a database system, users program according to sub-patterns. There is software mapping between submodes and modes and storage modes. Therefore, there are two levels of independence between programs and data: physical independence and logical independence. The storage mode of data changes, but the mode does not need to change, so there is no need to rewrite the application. This is called physical independence. When the schema changes, the sub-schema may not change, so the application does not need to be rewritten. This is called logical independence. Because the database system has a relatively high degree of independence between programs and data, it allows programmers to concentrate on algorithm logic when programming applications without having to worry about physical details, and can greatly reduce the workload of application maintenance.

Control characteristics The database data is large in quantity and complex in structure, and is shared by multiple users. Therefore, the database management system must conduct unified management and control during definition, establishment, operation, and maintenance to ensure the security, integrity, and consistency of concurrent operations of database data. In addition, there must be a database administrator who is responsible for the management, control, supervision and improvement of the database.

Because the database system has the above characteristics, its emergence has changed the development of information systems from centering on programs for processing data to centering on shared databases. This facilitates centralized management of data and is beneficial to the development and maintenance of applications. Data reduces redundancy and improves consistency, thereby improving consistency in decision-making. Therefore, most large-scale and complex information systems have databases as their core, and database systems play an increasingly important role in computer applications.

Research Topics Database research topics mainly involve three areas.

Development of database management system software DBMS is the foundation of the database system. The basic goal of developing a DBMS is to expand functions, improve performance and usability, thereby improving user productivity. Since the 1970s, the focus of research and development has been to explore the design of relational database management systems, including relational data language, query optimization, concurrency control and system performance. Another type of topic is the research on DBMS standardization, that is, the research on the specifications of a unified DBMS architecture.

Database design is based on the database management system of the computer system, and in accordance with the application requirements and the data model and functions provided by the computer system, designing a well-structured, easy-to-use, and high-efficiency database. An application information system with database as the core. The main research topic in this field is the exploration of database design methodology and design tools. For example, use software engineering methods and tools to guide database design;......>>

Question 2: Definition of database A database is, simply speaking, an electronic filing cabinet that can be viewed as an electronic file cabinet. ―A place where electronic files are stored. Users can add, intercept, update, delete, etc. the data in the files. Database refers to data that is stored together in a certain way, can be shared by multiple users, has as little redundancy as possible, and is independent of the application.

In the daily work of economic management, it is often necessary to put certain relevant data into such a "warehouse" and process it accordingly according to management needs. For example, the personnel department of an enterprise or public institution often stores the basic information of its employees (employee number, name, age, gender, place of origin, salary, resume, etc.) in a table. This 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, etc. If all these tasks can be automatically performed on computers, then our personnel management can reach an extremely high level. In addition, many such databases also need to be established in financial management, warehouse management, and production management, so that computers can be used to realize automated management of finance, warehouse, and production. A database is organized according to a certain data model and stores data in secondary storage***. This kind of data*** has the following characteristics: it is as non-duplicate as possible, serves multiple applications of a specific organization in an optimal way, its data structure is independent of the applications that use it, and it does not allow the addition, deletion, modification, or modification of data. Checks are managed and controlled by unified software. From the perspective of development history, database is an advanced stage of data management, which was developed from file management system.

Question 3: What does <>'' mean in the database? The inequality sign can also be written in some languages: # or !=

Question 4: What does @ mean in the database]@ ]@]

Represents local variables

Question 5: What does @ mean in the database]@]@]

Yes: local variable declaration, if there is no @ The field representative is the column name;

eg:

Declare variable: declare @name varchar(8)

Assign value: set @name= 'Zhang San'

Query: select * from stuInfo where stuName = @name

Assignment by set and select;

Select is generally used to query data and then assign variables .

Also

error and so on are global variables, which are customized by the system. We can only read them and cannot change them! !

Question 6: What is the meaning of code in database? Code is a basic concept in data system. The so-called code is an attribute that can uniquely identify an entity. It is a property of the entire entity set, not a property of a single entity. It includes super code, candidate code and main code. A hyperkey is a combination of one or more attributes that allow us to uniquely identify an entity in an entity set. If K is a supercode, then any superset of K is also a supercode. That is to say, if K is a supercode, then all *** containing K are also supercodes. The candidate code is selected from the super code. Naturally, the candidate code is also the *** of one or more attributes. Because the scope of supercode is too wide, many of them are not interesting to us and are useless. So candidate codes are minimal supercodes, and no proper subset of them can become a supercode. For example, if K is a supercode, then all *** containing K cannot be candidate codes; if K and J are not supercodes, then *** (K, J) composed of K and J may be candidate codes. It is to randomly select one of multiple candidate codes as the main code. If there is only one candidate code, then the candidate code is the main code. Although the choice of the main code is relatively arbitrary, in actual development, a certain amount of experience is still required, otherwise the developed system will have many problems. Generally speaking, the main code should choose attributes that never or rarely change. blog.csdn/...6.aspx

Question 7: Database, what does database mean? Data is an extremely important resource in today's information society, and all people's activities are inseparable from data. For example, student files in schools, academic performance records, school accounting, factory automation systems require real-time data on production lines, military commanders require various intelligence, and staff in various departments deal with data all the time. In the information age, due to the rapid growth of data volume, computers need to be used to process and process large amounts of data quickly and accurately, so databases and database management systems were produced. Simply put, a database (Data Base, abbreviated as DB) is a collection of records and files organized for a special purpose. Database management system (Data Base Management System, abbreviated as DBMS) is system software that defines, describes, establishes, manages, and maintains databases in computers.

A database system generally includes four important components: database, users, software and hardware.

Data refers to the centralized data stored in the database system, that is, the database, which is the working object of the database system. Usually, the data stored in the database is integrated and shared. sex.

Integration refers to the centralized storage of various application-related data and connections between data in a specific application environment according to a certain structure.

Sharability means that a piece of data in the database can be shared by multiple different users, that is, multiple different users, using multiple different languages, for different purposes Application purpose is to access the database at the same time, or even access the same piece of data at the same time.

User, there is a group of users who use the database, which refers to various requests for storing, maintaining and retrieving data. There are three types of users in a database system: end users, application programmers, and database administrators.

End users refer to people who access the database from computer terminals, and can also be called online users.

An application programmer is a person responsible for writing applications.

A database administrator refers to a person who is fully responsible for database access, maintenance and normal use.

Software refers to the software system responsible for database access, maintenance and management, usually called a database management system (DBMS). The various operation requests of various users on the database in the database system are all completed by DBMS, which is the core software of Su Opera Kuxiatai. DBMS works with the support of the operating system.

Question 8: *What does it mean in the database* means all

For example: select * from MyTable

It means to select all columns in the table MyTable Select the data

Question 9: What does (+) mean in the database statement? In Oracle, (+) is used to represent a left join.

and a.serv_id = b.serv_id( +) Equivalent to

a left join b on a.serv_id = b.serv_id

In turn

and a.serv_id(+) = b.serv_id That is a right join, which is equivalent to

a right join b on a.serv_id = b.serv_id

These two sentences are equivalent in Oracle. I have checked the following Plan, both consume the same time