Database three-level mode and Level two mapping introduction _ Database Other

Three-level schema and level two mappings for databases

The three-level schema of the database is mapped as shown in the following figure

Level three mode

External pattern: The logical structure of some of the data used by an application. A database can have multiple external patterns

Conceptual mode (schema, logical mode): The view that the database administrator uses is the visualization of our DBMS

The database object that is seen in the interface. A database has only one conceptual pattern www.jb51.net

Internal mode: The lowest layer mode of database is the description of data physical structure and storage mode. There is only one internal mode for a database.

Level Two mapping

Outside mode-mapping of conceptual patterns

Conceptual pattern-mapping of internal schemas

The relationship of level three mode

Mode (conceptual model) is the core and key of database

Internal mode relies on pattern, independent of external mode and storage device

External mode for specific applications, independent of internal mode and storage devices

Applications rely on external schemas, independent of schema and internal schemas

Two-level independence of the database

Physical independence: The application is independent of the data stored in the database on disk

Logical independence: The logical structure of applications and databases is independent of each other www.jb51.net

Note: Logical independence is harder to implement than physical independence

Level two independence relies on level two mappings

Here's a little bit of data model knowledge

Here you must note that the conceptual model in level three model corresponds to the logical model of database design, rather than the conceptual model (E-R model), must not be confused.