CHAPTER 2 Dbms

Question 1

WHAT IS ER DIAGRAM

Answer 1

• ER DIAGRAM IS ENTITY RELATIONSHIP DIAGRAM ALSO KNOWN AS ENTITY RELATIONSHIP MODEL
• ER DIAGRAM WAS DEVELOP TO GET GRAPHICAL REPRESENTATION THAT DEPICTS RELATIONSHIP AMONG ENTITY (OBJECT , PEOPLE…) OR CONCEPT THAT REPRESENT OVERALL LOGICAL STRUCTURE OF DATABASE

Question 2

ER MODEL

Answer 2

Entity-Relationship Model

Entity-Relationship (ER) Model is based on the notion of real-world entities and relationships among them. While formulating real-world scenario into the database model, the ER Model creates entity set, relationship set, general attributes and constraints.

Question 3

ENTITY AND RELATION SHIP

Answer 3

Entity − An entity in an ER Model is a real-world entity having properties called attributes. Every attribute is defined by its set of values called domain. For example, in a school database, a student is considered as an entity. Student has various attributes like name, age, class, etc.

Relationship − The logical association among entities is called relationship. Relationships are mapped with entities in various ways. Mapping cardinalities define the number of association between two entities.

Question 4

ENTITY

Answer 4

An entity can be a real-world object, either animate or inanimate, that can be easily identifiable. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity.

An entity set is a collection of similar types of entities. An entity set may contain entities with attribute sharing similar values. For example, a Students set may contain all the students of a school; likewise a Teachers set may contain all the teachers of a school from all faculties. Entity sets need not be disjoint.

Question 5

ATTRIBUTES

Answer 5

Entities are represented by means of their properties, called attributes. All attributes have values. For example, a student entity may have name, class, and age as attributes.

There exists a domain or range of values that can be assigned to attributes. For example, a student’s name cannot be a numeric value. It has to be alphabetic. A student’s age cannot be negative, etc.

Question 6

TYPES OF ATTRIBUTES

Answer 6

Simple attribute − Simple attributes are atomic values, which cannot be divided further. For example, a student’s phone number is an atomic value of 10 digits.

Composite attribute − Composite attributes are made of more than one simple attribute. For example, a student’s complete name may have first_name and last_name.

Derived attribute − Derived attributes are the attributes that do not exist in the physical database, but their values are derived from other attributes present in the database. For example, average_salary in a department should not be saved directly in the database, instead it can be derived. For another example, age can be derived from data_of_birth.

Single-value attribute − Single-value attributes contain single value. For example − Social_Security_Number.

Multi-value attribute − Multi-value attributes may contain more than one values. For example, a person can have more than one phone number, email_address, etc