Database systems have which three-level schema structure and its advantages?

Three-level schema structure: external schema, conceptual schema and internal schema \x0d\ I. Conceptual Schema (Schema) \x0d\ Definition: also known as the logical schema, is the logical structure of all the data in the database and the characterization of all the data is the public **** view of the data of all users. \x0d\ Understanding: ① a database has only one conceptual schema; \x0d\② is a view of the database data at the logical level; \x0d\③ database schema is based on a certain data model; \x0d\ second, the external schema (External Schema) \x0d\ Definition: also known as the sub-schema (Subschema) or the user schema, it is the 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 database user's view of the data, is a logical representation of the data related to a particular application. \x0d\ Understand: \1) a database can have more than one external schema; \x0d\ \2) the external schema is the user view; \x0d\ \3) the external schema is a powerful measure to ensure data security. \x0d\3, Internal Schema (Internal Schema) \x0d\ Understanding: ① a database has only one internal schema; \x0d\② a table may consist of multiple files, such as: data files, index files. \x0d\ It is a database management system (DBMS) to effectively organize and manage the data in the database its purpose are: ① in order to reduce data redundancy, to achieve the data *** enjoyment; \x0d\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ to increase access efficiency, improve performance.