Concepts answers

What is database? Q & Answers A database is a logically coherent collection of data with some inherent meaning, representing some aspect of real world and which is designed, built and populated with data for a specific purpose. 2. What is DBMS? It is a collection of programs that enables user to create and maintain a database. In other words it is general-purpose software that provides the users with the processes of defining, constructing and manipulating the database for various applications. 3. What is a Database system?

The database and DBMS software together is called as Database system. 4. Advantages of DBMS? Redundancy is controlled. Unauthorised access is restricted. Providing multiple user interfaces. Enforcing integrity constraints. Providing backup and recovery. 5. Disadvantage in File Processing System? Data redundancy & inconsistency. Difficult in accessing data. Data isolation. Data integrity. Concurrent access is not possible. Security Problems. 6. 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 nd what relationship among those data. View level: The highest level of abstraction describes only part of entire database. 7. 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. 8. What is extension and intension? Extension – It is the number ot 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 n it. 9. What is System R? What are its two major subsystems? 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 lifeenvironmentto solve real life problems, with performance at least comparable to that of existing system. Its two subsystems are Research Storage System Relational Data System. 0. How is the data structure of System R different from the relational structure? 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 11. What is Data Independence? Data independence means that ” the application is independent of the storage structure and access strategy of data”. In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.

Two types of Data Independence: Physical Data Independence: Modification in physical level should not affect the logical level. Logical Data Independence: Modification in logical level should affect the view level. NOTE: Logical Data Independence is more difficult to achieve 12. What is a view? How it is related to data independence? A view may be thought of as a virtual table, that is, a table that does not really exist in its own right but is instead derived from one or more underlying base table. In other words, there is no stored file that direct represents the view instead a definition of view is stored in data dictionary.

Growth and restructuring of base tables is not reflected in views. Thus the view can insulate users from the effects of restructuring and growth in the database. Hence accounts for logical data independence. 3. What is Data Model? A collection of conceptual tools for describing data, data relationships data semantic cs and constraints. 14. What is E-R model? This data model is based on real world that consists of basic objects called entities and of relationship among these objects. Entities are described in a database by a set ot attributes. 15. What is Object Oriented model? This model is based on collection of objects.

An object contains values stored in instance variables with in the object. An object also contains bodies of code that operate on the object. These bodies of code are called methods. Objects that contain ame types of values and the same methods are grouped together into classes. 16. What is an Entity? It is a ‘thing’ in the real world with an independent existence. 17. What is an Entity type? It is a collection (set) of entities that have same attributes. 18. What is an Entity set? It is a collection of all entities of particular entity type in the database. 19. What is an Extension of entity type?

The collections of entities of a particular entity type are grouped together into an entity set. 20. What is Weak Entity set? An entity set may not have sufficient attributes to form a primary key, and its primary ey compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set. 21 . What is an attribute? It is a particular property, which describes the entity. 22. What is a Relation Schema and a Relation? A relation Schema denoted by R(AI, A2, … , An) is made up of the relation name R and the list of attributes Ai that it contains. A relation is defined as a set of tuples.

Let r be the relation which contains set tuples (tl, t2, t3, tn). Each tuple is an ordered list of n-values t=(v1, v2, vn). 23. What is degree of a Relation? It is the number of attribute of its relation schema. 24. What is Relationship? It is an association among two or more entities. 25. What is Relationship set? The collection (or set) of similar relationships. 26. What is Relationship type? Relationship type detlnes a set ot associations or a relationship set among a given set of entity types. 27. What is degree of Relationship type? It is the number of entity type participating.