Describe the three levels of data abstraction?

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"

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.

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RDBMS