DBMS

21. What are composite attributes? 

Composite attributes can be divided in to sub parts. 

22. Define null values. 

In some cases a particular entity may not have an applicable value for an attribute or if we do not know the value of an attribute for a particular entity. In these cases null value is used. 

23. Define the terms 

i) Entity type 

ii) Entity set 

Entity type: An entity type defines a collection of entities that have the same attributes. 

Entity set: The set of all entities of the same type is termed as an entity set. 

24. What is meant by the degree of relationship set? 

The degree of relationship type is the number of participating entity types. 

25. Define the terms 

i) Key attribute 

ii) Value set 

Key attribute: An entity type usually has an attribute whose values are distinct from each individual entity in the collection. Such an attribute is called a key attribute. 

Value set: Each simple attribute of an entity type is associated with a value set that specifies the set of values that may be assigned to that attribute for each individual entity. 

26. Define weak and strong entity sets? 

Weak entity set: entity set that do not have key attribute of their own are called weak entity sets. 

Strong entity set: Entity set that has a primary key is termed a strong entity set. 

27. What does the cardinality ratio specify? 

Mapping cardinalities or cardinality ratios express the number of entities to which another entity can be associated. Mapping cardinalities must be one of the following: 

• One to one 

• One to many 

• Many to one 

•Many to many 

28. Explain the two types of participation constraint. 

•Total: The participation of an entity set E in a relationship set R is said to be total if every entity in E participates in at least one relationship in R. 

• Partial: if only some entities in E participate in relationships in R, the participation of entity set E in relationship R is said to be partial. 

29. Define the terms 

i) DDL 

ii) DML 

DDL: Data base schema is specified by a set of definitions expressed by a special language called a data definition language. 

DML: A data manipulation language is a language that enables users to access or manipulate data as organized by the appropriate data model. 

30. Write short notes on relational model 

The relational model uses a collection of tables to represent both data and the relationships among those data. The relational model is an example of a record based model.