Data Abstraction, System R and Integrity Rules

Describe the three levels of data abstraction?
ANSWER:
The are three levels of abstraction:
• Physical level: The lowest level of abstraction describes how data are stored.
• Logical level: The next higher level of abstraction, describes what data are stored in database and what relationship among those data.
• View level: The highest level of abstraction describes only part of entire database.

Define the "integrity rules"
ANSWER:
There are two Integrity rules.
• Entity Integrity: States that “Primary key cannot have NULL value”
• Referential Integrity: States that “Foreign Key can be either a NULL value or should be Primary Key value of other relation.

What is extension and intension?
ANSWER:
Extension -It is the number of tuples present in a table at any instance. This is time dependent.
Intension - It is a constant value that gives the name, structure of table and the constraints laid on it.

What is System R? What are its two major subsystems?
ANSWER:
System R was designed and developed over a period of 1974-79 at IBM San Jose Research Center . It is a prototype and its purpose was to demonstrate that it is possible to build a Relational System that can be used in a real life environment to solve real life problems, with performance at least comparable to that of existing system.
Its two subsystems are
• Research Storage
• System Relational Data System.

How is the data structure of System R different from the relational structure?
ANSWER:
Unlike Relational systems in System R
• Domains are not supported
• Enforcement of candidate key uniqueness is optional
• Enforcement of entity integrity is optional
• Referential integrity is not enforced