Introduction to three-level database modes and two-level ing

Three-level database mode and two-level ing

Shows the two-level ing of the database's three-level mode.

Level 3 mode

External mode: ApplicationProgramThe logical structure of some data used. One database can have multiple external Modes

Concept mode (mode and logic mode): The view used by the database administrator is the visualization of the DBMS.

The database object displayed on the page. A database has only one conceptual model www.2cto.com.

Internal Mode: the lowest layer mode of the database, which is the description of the physical structure and storage mode of the data. A database has only one internal mode.

Two-level ing

External mode-concept mode ing

Concept mode-Internal Mode ing

Relationship between level-3 models

Mode (concept mode) is the core and key of the database

The internal mode depends on the mode and is independent of the external mode and storage device.

The external mode is applicable to specific applications and is independent of the internal mode and storage device.

The application depends on the external mode and is independent from the internal mode.

Two levels of database independence

Physical independence: Data in applications and databases stored on disks is independent of each other.

Logical independence: the logical structure of applications and databases is independent of each other.

Note: Logical independence is more difficult to implement than physical independence.

Two-level independence depends on two-level ing.

The following is a bit of data model knowledge.

Here we must pay attention to the three-level mode of the concept of the corresponding logical model of the database design, rather than the concept model (E-R model), must not confuse.