Lecture 1 pt.2 Flashcards
(35 cards)
A set of concepts to describe the structure of a database, the operations for manipulating these structures, and certain constraints that the database should obe
Data Model
Constructs define database structure.
Include elements (with data types) and groups (e.g., entity, record, table) with relationships.
Enforce data validity restrictions continuously.
Data Model Structure and Constraints
These operations are used for specifying database retrievals and updates by referring to the constructs of the data model
Data Model Operations
Categories of Data Models:
Provide concepts that are close to the way many users perceive data.
(Also called entity-based or object-based data models.)
Conceptual (high-level, semantic) data models
Categories of Data Models:
Provide concepts that describe details of how data is stored in the computer. These are usually specified in an ad-hoc manner through DBMS design and administration manuals
Physical (low-level, internal) data models
Categories of Data Models:
Provide concepts that fall between physical and conceptual data models, used by many commercial DBMS implementations (e.g. relational data models used in many commercial systems).
Implementation (representational) data models
The description of a database. Includes descriptions of the database structure, data types, and the constraints on the database
Database Schema
An illustrative display of (most aspects of) a database schema
Schema Diagram
A component of the schema or an object within the schema, e.g., STUDENT, COURSE
Schema Construct
The actual data stored in a database at a particular
moment in time. This includes the collection of all the data in the database
Database State
Also called database instance (or occurrence or snapshot).
Database State
Refers to the database state when it is initially loaded into the system
Initial Database State
A state that satisfies the structure and constraints of the database
Valid State
is also called intension
Schema
is also called extension
State
changes very infrequently
database schema
changes every time the database is updated
database state
- Supports DBMS characteristics
- Program-data independence.
- Support for multiple data views.
- Not widely implemented in commercial DBMS products.
- Useful for explaining database system organization
Three-Schema Architecture
Defines DBMS schemas at three levels
Three-Schema Architecture
Three-Schema Architecture includes
Internal Schema
Conceptual Schema
External Schema
describes physical storage structures and access paths (e.g indexes). Typically uses a physical data mode
Internal schema
describes the structure and constraints for the whole database for a community of users.
Conceptual schema
describes the various user views. Usually uses the same data model as the conceptual schema
External schemas
The capacity to change the conceptual schema without having to change the external schemas and their associated application programs.
Logical Data Independence
