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This article mainly explains "what is the three-level mode and two-level mapping of the database". Interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let's let the editor take you to learn "what is the three-level mode and two-level mapping of the database?"

Three-level schema and two-level mapping of database

The three-level schema and two-level mapping of the database is shown in the following figure

Three-level mode

External schema: the logical structure of part of the data used by the application. A database can have multiple external schemas

Conceptual schema (schema, logical schema): the view used by the database administrator is our visualization in DBMS

The database objects seen in the interface. A database has only one conceptual schema www.jb51.net

Internal schema: the lowest-level schema of a database, which is a description of the physical structure and storage of the data. A database has only one internal schema.

Two-level mapping

Mapping of external schema to conceptual schema

Mapping of Conceptual schemas to Inner schemas

The relationship of the three-level model

Schema (Conceptual Schema) is the core and key of database

The inner mode is dependent on the pattern, independent of the external mode and storage device.

The external mode is oriented to specific applications, independent of the internal mode and storage devices.

The application depends on the external schema, independent of the schema and the internal schema

Two-level independence of database

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

Logical independence: the logical structure of the application and the database are independent of each other www.jb51.net

Note: logical independence is more difficult to achieve than physical independence.

Two-level independence depends on two-level mapping

The following is to add a little knowledge of the data model

Here, we must note that the conceptual schema in the three-level schema corresponds to the logical model of database design, not the conceptual model (Emurr model), which must not be confused.

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