The three-level pattern is the three levels of abstraction of the data, each of which focuses on different concerns. In order to associate the three-level patterns, the database management system provides the second-level image function between the three-level patterns. A secondary image is a rule that dictates how both sides of the image can be transformed. As shown below:
Three-level mode
User-level --> out-of-schema (reflects the user view of the database system)
Outer mode, also known as submode or user mode, corresponds to the user level. It is a view of the database as seen by one or a few users, and is a logical representation of data related to an application. An outer schema is a subset of the schema that is derived from a schema and contains the portion of data that is allowed to be used by a specific user in the schema. Users can describe and define data records corresponding to users through the outer mode description language (outer pattern), and can also use the Data Manipulation Language (DML) to manipulate these data records.
Conceptual level - > Conceptual pattern (reflects the holistic view of the database system)
Conceptual patterns, also known as patterns or logical patterns, correspond to conceptual levels. It is a global logical structure constructed by the database designer to synthesize the data of all users according to a unified point of view, an overall description of the logical structure and characteristics of all data in the database, and a public data view (global view) for all users. It is described and defined by the Data Description Language (DDL) provided by the database management system.
Physical level -- in-> schema (reflects the storage view of the database system)
Inner mode, also known as storage mode, corresponds to the physical level. It is the internal representation or underlying description of all data in the database, and is the lowest level of logical description of the database, which describes the storage method and physical structure of the data on the storage medium, corresponding to the database actually stored on the external storage medium. The inner pattern is described and defined by the internal pattern description language.
Secondary Image (Mapping)
The database system provides two levels of images between the three levels of mode: the image of the mode/inner pattern, and the image of the outer pattern/pattern.
- Image of Pattern/Inner Pattern: Implements interconversion between conceptual patterns and inner patterns.
- Outer Mode/Mode Image: Implements interconversion between Outer Mode and Conceptual Mode.
Data independence refers to the independence of data and programs, separating the definition of data from the program, and the DBMS (database management system) is responsible for data storage, thereby simplifying the application and greatly reducing the workload of application preparation.
Data independence is ensured by the DBMS's secondary image capabilities. Data independence includes the physical independence of data and the logical independence of data.
Summary, as shown in the figure below:
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Posted on 5/19/2024 12:28:22 PM
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