The architecture of the database system refers to the structure of the entire system of the database system. The architecture of a database system can be divided in different ways from different perspectives. From the perspective of the database management system, it can be divided into three layers, from outside to inside, they are external schema, schema and internal schema. The schema is a common data view for all database users and a description of the logical structure and characteristics of all data in the database. Schema can be subdivided into conceptual schema and logical schema.
##1.Three-level schema structure of the database
The three-level schema structure of the database system refers to the schema, the outer schema, and the inner schema. The following are introduced respectively:Pattern
Pattern is also called logical pattern or conceptual pattern, which is a description of the logical structure and characteristics of all data in the database. . Is a public view of the data for all users. A database has only one schema. Patterns are at the middle level of the three-level structure. When defining a schema, you must not only define the logical structure of the data, but also define the connections between the data, and define the security and integrity requirements related to the data.External mode
External mode is also called user mode. It is what database users (including application programmers and end users) can see and use. The description of the logical structure and characteristics of the local data is the data view of the database user and the logical representation of the data related to a certain application. Foreign schemas are a subset of schemas, and a database can have multiple schemas.Internal schema
The internal schema is also called the storage schema. A database has only one internal schema. It is a description of the physical structure and storage method of data, and the representation of data within the database.Mapping between second and third level modes
In order to be able to internally To realize the connection and conversion of the three abstract levels of the database, the database management system provides two levels of mapping between the three-level schemas, namely external schema/schema mapping and schema/internal schema mapping. They are introduced below:Foreign schema/schema mapping
There can be any number of foreign schemas for the same schema. For each foreign schema, the database system has a foreign schema/schema mapping. When the schema is changed, the database administrator makes corresponding changes to each foreign schema/schema mapping so that the foreign schema remains unchanged. In this way, applications written based on data external schema do not need to be modified, ensuring the logical independence of data and programs.
Schema/inner schema mapping
The database has only one schema and one inner schema, so the schema/inner schema mapping is unique, and it defines the global logic of the database Correspondence between structures and storage structures. When the storage structure of the database is changed, the database administrator can make corresponding changes to the schema/internal schema mapping, so that the schema remains changed, and the application program does not change accordingly. This ensures the independence of data and programs.
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