CMPSC 431W Lecture Notes - Lecture 2: Logical Data Model, Physical Data Model, Relational Model
Document Summary
Example: students (sid: string, name: string, login: string, age: integer, gpa: real) External schema: describes how user see the data. Allows data access to be customized at the level of individual users or groups of users. Conceptual schema: defines logical structures based on data models. Describes the stored data in terms of the data model of the dbms. Physical schema: defines the files and indexes used. Summarizes how the relations described in the conceptual schema are actually stored on secondary storage devices such as disks and tapes: example: relations stored as unordered files, example: second column of students is indexed by a b-tree. We want different levels of abstraction because we want data independence. Logical data independence: protection from changes in logical structure of the data. Ability to change the logical schema without having to change the external schema. Physical data independence: protection from changes in physical structure of data. Change the index from hash to b-tree.
