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Er model

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The ER model defines the conceptual view of a database. It works around real-world
entities and the associations among them. At view level, the ER model is considered a
good option for designing databases.
Entity
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.
Attributes
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.
Types of Attributes
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.

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These attribute types can come together in a way like −
simple single-valued attributes
simple multi-valued attributes
composite single-valued attributes
composite multi-valued attributes
Entity-Set and Keys
Key is an attribute or collection of attributes that uniquely identifies an entity among
entity set.
For example, the roll_number of a student makes him/her identifiable among students.
Super Key − A set of attributes (one or more) that collectively identifies an entity
in an entity set.
Candidate Key A minimal super key is called a candidate key. An entity set
may have more than one candidate key.
Primary Key A primary key is one of the candidate keys chosen by the
database designer to uniquely identify the entity set.
Relationship
The association among entities is called a relationship. For example, an
employee works_at a department, a student enrolls in a course. Here, Works_at and
Enrolls are called relationships.
Relationship Set
A set of relationships of similar type is called a relationship set. Like entities, a
relationship too can have attributes. These attributes are called descriptive attributes.
Degree of Relationship
The number of participating entities in a relationship defines the degree of the
relationship.
Binary = degree 2
Ternary = degree 3
n-ary = degree
Mapping Cardinalities

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The ER model defines the conceptual view of a database. It works around real-world entities and the associations among them. At view level, the ER model is considered a good option for designing databases. Entity 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. Attributes 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. Types of Attributes • 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 at ...
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