Database System (DBS)
Refers to the system
function after the computer is introduced into the database : it can store a large amount of data in an organized and dynamic manner, and provide a data processing and data sharing mechanism.
DBS = computer system (hardware, software platform, people) + DBMS + DB
Data management:
It is a technology that studies how to classify, organize, encode, store, retrieve, and maintain data.
Three stages of data management : manual management, file system management, and database system management (this stage has the highest independence)
manual management:
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Data cannot be stored for a long time
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Data cannot be shared, with great redundancy
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Poor data independence
File system management: -
Poor logical independence
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Data redundancy is greater
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File application programming is complicated
The characteristics of the database system:
- Data structuring: Oriented to the whole organization, with overall structure, and data connections
- High data sharing, low redundancy, and easy to expand
- High data independence: divided into physical independence and logical independence.
Physical independence -----> Application and data (physical storage) are independent.
Logical independence -----> Application and logical structure are independent of
DBMS Provide data security, integrity, concurrency control, database recovery,
database system three-level model structure
Mode, sub mode, inner mode definition
- Mode : It is the characteristic description of the logical structure of all data in the database, and is the common database structure for all users ( a database has only one mode )
- Sub-mode : The description of the logical structure and characteristics of the local data seen and used by the database user is the database structure used by the user.Sub-modeIt is a subset of the pattern. ( A database can have multiple sub-modes )
- Internal mode (storage mode): a description of the physical structure and storage method of the database. ( A database has only one internal mode )
External mode------mode------internal mode
Data independence and two-level image function
>Data independence:Refers to the independence of data and programs
Physical independence: Means that the change of the physical structure of the database does not affect the logical structure and applications (for example: the replacement of storage devices, the displacement of stored data...)
Logical independence: Means that the change in the logical structure of the database does not affect the application (for example: modify the definition of the data structure, add new data types...)
>Two-level image:
<Sub-mode/mode image>: The image is not unique, and the mode changes ----> The sub-mode remains unchanged to ensure logical independence.
<Mode/Intra-mode image>: The image is unique and guarantees the
three-level mode of physical independence . The advantages of the structure and the two-layer image
1. Ensure data independence
2. It is convenient for users to use and simplify the user interface.
Three, a powerful measure to ensure the security of the database.
4. Conducive to data sharing
5. Conducive to understanding the internal structure of the database system from a macro perspective.
Database system architecture
Client/server (c/s) structure [with client]
browser/server (B/S) structure [without client]