Traditional Culture Encyclopedia - Traditional festivals - What is the three-level schema of a database (What do each of the three levels of schema of a database system refer to?)
What is the three-level schema of a database (What do each of the three levels of schema of a database system refer to?)
2. What are each of the three levels of schema for a database system.
3, The three-level schema of a database is.
4, What are the three-level schema of a database.
1. Three-level schema structure: external schema, data schema, internal schema, schema definition: also known as the logical schema, is the description of the logical structure and characteristics of all the data in the database, is the public **** data view of all users, understanding: a database has only one schema, is the view of the database data at the logical level.
2. Database schema based on a particular data model, define the schema not only to define the logical structure of the data, but also to define and data-related security, integrity requirements, define the link between these data.
3. External model definition: also known as sub-model or user mode, is the database 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, and an application related to the logical representation of the data.
4. Understanding: a database can have more than one external schema, the external schema is the user view, the external schema is a powerful measure to ensure data security.
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