Database-DATABASE-table
SQL-database language
Database administrator-DBA
Database Management System-DBMS
Database application-DBAP
Database (DataBase):
A warehouse for storing data. This warehouse is on a computer storage device and the data is stored in a certain format. The database is an integration of multiple application data with a unified structure and stored in the same storage medium, and can be shared by various applications.
Database Management System (DataBase Mangement System):
A system software responsible for data organization, data manipulation, data maintenance, control, and data protection services in the database. It is the core of the database system.
The main function:
1. Data model definition
2. The physical construction of data access
3. Data manipulation
4. Data integrity, security definition and inspection
5. Concurrency control and failure recovery of data sets
In order to complete basic functions, the database management system will provide corresponding data language
Data Definition Language (DDL)
Data Manipulation Language (DML)
Data Control Language (DCL)
database system
Database management system refers to a computer system that has installed and used database technology
It includes database, database management system, application system, database administrator (DBA)
Responsibilities of DBA:
Database installation, database configuration and management, permission setting and security management, monitoring and performance adjustment, backup and recovery, solve half of the problems
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Characteristics of database system
Data integration
High data sharing and low redundancy
Data independence
Unified data management and control
Database system structure
From a database management system perspective: the database usually adopts a three-level structure, which is the internal architecture of the database management system
From the perspective of database end users: the structure of the database system is divided into single-user structure, master-slave structure, distributed structure, client\server structure, which is the external architecture of the database system
The three-level model structure of the database means that the database system is composed of three levels: external mode, mode and internal mode
mode
The description of the logic structure and characteristics of all three levels in the database is a common data view for all users. It is the middle layer of the database system schema structure, does not involve the physical storage details of the data and the hardware environment, and has nothing to do with specific applications and high-level programming languages.
In fact, the schema is a logical view of database data, and there is only one schema for a database. The database considers the needs of all users and integrates these needs into a logical whole.
Outer mode
External mode is also called sub mode or user mode. It is a description of the logical structure and characteristics of local data seen and used by database users (including application programmers and end users). It is a data view of database users and is related to a certain application. The logical representation of the data.
The external mode is a subset of the mode. A database has multiple external modes. Because it is the data view of each user, if different users have differences in application requirements, the way they view data, and the requirements for data confidentiality, they The description of the outer mode is different, even for the same data in the mode, the structure, type, length, and security level in the outer mode can be different. On the other hand, the same external mode can be used by multiple application systems of a user, but one application system can only correspond to one external mode.
The external mode is a powerful measure to ensure the security of the database. Each user can only see and access the data in the corresponding external mode, and the rest of the data in the database is invisible to them.
Inner mode
The internal mode is also called the storage mode, which is a description of the physical structure of the data and the storage structure. Is the representation of data inside the database. For example, is the storage method of records stored sequentially, stored in a B-tree structure or stored in a hash method; in what way the index is organized; whether the data is compressed and stored and encrypted; what are the requirements for the storage record structure of the data, etc. A database has only one internal mode.
The data model gives the data frame structure of the database. The data is the real entity in the database, but these data must be organized according to the structure described by the framework. The database composed of the conceptual model is called the conceptual database (Conceptual DataBase). The database composed of the framework is called the user database (User's Database), and the database composed of the internal mode is called the physical database. Among these three kinds of databases, only the physical database actually exists in the external memory of the computer, and the other two databases do not really exist in the computer, but are mapped from the physical database through two mappings.
The three levels of the model reflect the three different environments of the model and their different requirements. The internal model is at the lowest level, which reflects the actual storage form of data in the computer physical structure, and the conceptual model is at the middle level, which reflects the design The user’s data global logic requirements, while the outer mode is the outermost layer, which reflects the user’s data requirements.
Database secondary image function
1. The three-level model of the database system is three abstract levels of data. It leaves the specific organization of data to the database management system (DBMS) management, enabling users to process data logically and abstractly, without having to care about the specific representation and storage of data in the computer.
2. In order to realize the connection and conversion of these three levels of abstraction internally, the database system provides two layers of mapping between these three levels of mode: external mode/mode mapping and mode/internal mode mapping. It is these two layers of mapping that ensure that the data in the database system can have a high logical and physical independence.
External mode/mode image
1. The mode describes the global logical structure of the data, and the outer mode describes the local logical structure of the data. There can be any number of outer modes corresponding to the same mode. For each external mode, the database system has an external mode/mode image, which defines the corresponding relationship between the external mode and the mode.
2. When the mode changes, the database administrator will make corresponding changes to each external mode/mode image, and the external mode can also be kept unchanged, because the application program is written according to the external mode of the data, so the application program does not need to be modified. Ensure the logic independence of data and program.
Mode/intra mode image
1. Mode/intra mode mapping defines the correspondence between the global logical structure of the data and the physical storage structure. When the storage structure of the database changes (for example, another disk is used to store the database), the database administrator can make corresponding changes to the schema/internal schema image to keep the schema unchanged, thereby ensuring the physical independence of the data.
