DBMS Chapter 2 Flashcards
(49 cards)
Data abstraction
The suppression of details and highlighting of the essential features
Data Model
A set of concepts to describe the structure of a database, and certain constraints that the database should obey.
Client Module
Designed so that it will run on a user workstation or PC
Server Module
Handles data storage, access, search functions
Categories of data models
Conceptual (high-level, semantic) data models:
Physical (low-level, internal) data models:
Implementation (representational) data models:
Conceptual (high-level, semantic) data models
Provide concepts that are close to the way many users perceive data.
Physical (low-level, internal) data models
Provide concepts that describe details of how data is stored in the computer
Implementation (representational) data models
Provide concepts that fall between the above two, balancing user views with some computer storage details.
Advantage of Hierarchical Model
- Hierarchical Model is simple to construct and operate on
- Corresponds to a number of natural hierarchically organized domains
- Language is simple
Disadvantage of Hierarchical Model
- Navigational and procedural nature of processing
- Database is visualized as a linear arrangement of records
- Little scope for “query optimization”
Advantages of Network Model
- Network Model is able to model complex relationships.
- Can handle most situations for modeling using record types and relationship types.
Disadvantage of Network Model
Navigational and procedural nature of processing
Database contains a complex array of pointers that thread through a set of records.
Little scope for automated “query optimization”
Database Schema
The description of a database. Includes descriptions of the database structure and the constraints that should hold on the database.
Schema Diagram
A diagrammatic display of (some aspects of) a database schema.
Schema Construct
A component of the schema or an object within the schema
Database Instance
The actual data stored in a database at a particular moment in time
Database State
Refers to the content of a database at a moment in time.
Valid State
A state that satisfies the structure and constraints of the database.
Database schema vs. Database state
- The database schema changes very infrequently. The database state changes every time it’s updated
- Schema is also called intension, whereas state is called extension.
3-schema Architecture
Internal schema, Conceptual schema, External schema
Internal schema
Describes physical storage structures and access paths. Typically uses a physical data model.
Conceptual schema
Describes the structure and constraints for the whole database for a community of users. Uses a conceptual or an implementation data model.
External schema
Describes the various user views. Usually uses the same data model as the conceptual level.
Mappings
Processes of transforming requests and results between levels
