What describes the global logical structure and characteristics of all data in the database is the "schema". Schema, also called logical schema, is a description of the logical structure and characteristics of all data in the database and is a common data view for all users; the database system consists of external schema, schema and internal schema.
The operating environment of this tutorial: windows7 system, mysql8 version, Dell G3 computer.
In the three-level schema of the database, the "schema" describes the global logical structure and characteristics of all data in the database.
People have designed a rigorous architecture for the database. The standard structure recognized in the database field is the three-level schema structure, which includes external schema, conceptual schema, and internal schema to effectively organize and manage data. Improved the logical independence and physical independence of the database.
Schema, also called logical schema, is a description of the logical structure and characteristics of all data in the database, and is a common data view for all users. The user level corresponds to the external schema, the conceptual level corresponds to the conceptual schema, and the physical level corresponds to the internal schema, allowing users at different levels to form different views of the database.
The so-called view refers to the scope, angle and method of observing, understanding and understanding the data. It is the reflection of the database in the "eyes" of the user. Obviously, the database "seen" by users at different levels (levels) are not the same.
External mode
External mode is also called submode or user mode, corresponding to the user level. It is a data view of the database seen by one or several users, and is a logical representation of data related to a certain application. The foreign schema is a subset derived from the schema and contains that portion of the data in the schema that is allowed to be used by a specific user. Users can describe and define data records corresponding to users (external schemas) through external schema description languages, and they can also use data manipulation language (Data Manipulation Language, DML) to operate on these data records. The external schema reflects the user's view of the database system.
Conceptual pattern
Conceptual pattern is also called pattern or logical pattern, corresponding to the concept level. It is a global logical structure constructed by the database designer based on a unified point of view, integrating the data of all users. It is an overall description of the logical structure and characteristics of all data in the database, and is a common data view (global view) for all users. It is described and defined by the Data Description Language (DDL) provided by the database management system. Conceptual schema reflects the overall view of the database system.
Internal mode
Internal mode is also called storage mode, which corresponds to the physical level. It is the internal representation or underlying description of all data in the database, and is the lowest level logical description of the database. It describes the storage method and physical structure of the data on the storage medium, and corresponds to the database actually stored on the external storage medium. Internal patterns are described and defined by internal pattern description languages. The internal schema reflects the storage view of the database system.
In a database system, there is only one database. Therefore, as the internal schema that defines and describes the database storage structure and the schema that describes the logical structure of the database, it is also unique, but it is built on the database system. The applications are very extensive and diverse, so the corresponding external model is not unique, nor can it be unique.
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