Unit 1 - DBMS

21. Define the terms i) Entity set ii) Relationship set

Relationship set : The set of all relationships of the same type is termed as a relationship set.

22. Define single valued and multivalued attributes.

Single valued attributes: attributes with a single value for a particular entity are called single

valued attributes.

Multivalued attributes : Attributes with a set of value for a particular entity are called

multivalued attributes.

23. What are stored and derived attributes?

Stored attributes: The attributes stored in a data base are called stored attributes.

Derived attributes: The attributes that are derived from the stored attributes are called derived attributes.

24. What are composite attributes?

Composite attributes can be divided in to sub parts.

25. 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.

26. 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.

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

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

28. 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.

29. 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.

30. 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