18 Questions
What does a physical data model provide concepts for?
How data is stored in the computer
Which category of data model provides concepts that are close to the way many users perceive data?
Conceptual data models
What is the purpose of a database schema?
Description of the database structure
What is another term for a database state?
Database instance
Which type of data model falls between conceptual and physical data models?
Implementation data models
What does a schema construct represent?
A component of the schema or an object in the schema
What is the difference between database schema and database state?
Schema is intension, while state is extension.
What is the purpose of the internal schema in a Three-Schema Architecture?
To describe the physical storage structures and access paths.
Which level of the Three-Schema Architecture focuses on describing the overall structure and constraints of the entire database?
Conceptual schema
How do users and programs interact with a Three-Schema Architecture in terms of schema levels?
They refer to an external schema which is then mapped by the DBMS to the internal schema for execution.
What is the purpose of defining DBMS schemas at three levels in a Three-Schema Architecture?
To provide program-data independence and support multiple views of the data.
Which term refers to a state that satisfies the structure and constraints of the database?
"Valid State"
What is the purpose of Data Definition Language (DDL) in a DBMS?
To specify the conceptual schema of a database
What is the main advantage of Logical Data Independence in a DBMS?
Avoiding changes in external schemas when internal schema changes
Which level of data independence allows changes in the internal schema without affecting the conceptual schema?
Physical Data Independence
In a DBMS, what is the function of View Definition Language (VDL)?
Defining internal and external schemas (views)
Why do application programs not need to be changed when lower-level schemas are modified in a DBMS?
Due to the independent nature of higher-level schemas from lower-level schemas
Which language is used by the DBA and database designers to define internal and external schemas in many DBMSs?
Data Definition Language (DDL)
Study Notes
Data Independence
- Logical Data Independence: ability to change the conceptual schema without changing the external schemas and their associated application programs
- Physical Data Independence: ability to change the internal schema without changing the conceptual schema
DBMS Languages
- Data Definition Language (DDL): used by DBA and database designers to specify the conceptual schema of a database
- Can also be used to define internal and external schemas (views)
- Theoretically, separate storage definition language (SDL) and view definition language (VDL) can be used to define internal and external schemas
Database State
- Initial Database State: database state when it is initially loaded into the system
- Valid State: a state that satisfies the structure and constraints of the database
- Database schema changes infrequently, while database state changes every time the database is updated
- Schema is also called intension, while state is also called extension
Three-Schema Architecture
- Proposed to support DBMS characteristics of program-data independence and support of multiple views of the data
- Defines DBMS schemas at three levels:
- Internal schema: describes physical storage structures and access paths (e.g. indexes)
- Conceptual schema: describes the structure and constraints for the whole database for a community of users
- External schemas: describe the various user views
- Mappings among schema levels are needed to transform requests and data
Categories of 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, used by many commercial DBMS implementations
Database Schema versus Database State
- Database Schema: description of a database, including descriptions of the database structure, relationships, data types, and constraints
- Database State: the actual data stored in a database at a particular moment in time, also called a database instance, occurrence, or snapshot
Explore concepts related to database state and schema, including initial database state, valid state, three-schema architecture, and their distinctions. Learn about the characteristics of the three-schema architecture in database management systems.
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