Er diagram notation

Er diagram:

An Er diagram is called an Entity-relationship model. It is a representation of data in picture form.

 

Er diagram notation:

1) Entity:

A real-world with an independent actuality. It can be an object with physical existence are with conceptual existence. Here in the below diagram Student is the entity.

2) Attributes:

Properties that describe the entities are called attributes. E.g:– In the above diagram Empname, Empid, and Agency are the attributes.

Types of attributes are

• • Simple attributes: Simple attributes cannot be divided further Example:- weight.
  • Composite attributes: It can be divided further. For example, the name can be divided into first name, middle name, and last name.
  • Single-valued attributes: Single-valued attributes have only one value for a particular entity. For example, age single-valued attribute of a person.
  • Multivalued attributes: Multivalued attributes can have a set of values for a particular entity. For example, a college degree, languages knew multivalued attributes of a person.

• • Derived attributes: Derived attributes can be derived from other attributes.
  • Stored attributes: Stored attributes from which the value of other attributes are derived. Example:- birth date of a person.
  • Complex attributes: Complex attributes has multivalued and composite components in them.

3) Null values:

Null is something that is not applicable is unknown.

student ID	age	name	phone number
1	18	Harry	6555455621
2	15	Ron	NULL
3	16	Joshua	6455554545

 

4) Entity types:

An entity with the same attributes.

EX: STUDENT

Student id	age	name
1	18	Harry
2	15	Ron
3	16	Joshua

5) Entity set:

A collection of entity types at a point in time. Where E represents the entity set.

A weak entity set is denoted as:

6) Key attribute:

The attribute that is capable of identifying each entity uniquely.

Ex: Student id of the student in the above table.

7) Value set of attributes:

The set of values that can be assigned to an attribute.

8)  Relationship set:

9) Weak relationship set:

Relationships in er diagram:

Relationships:

A relationship is an association or a connection or a link among two or more entities.

Degree of relationship:

It denotes the number of entity types that participate in a relationship.

Unary relationship:

A relationship that tells how many entities are associated or linked together relationship types of degree one is called unary relationships.

 

Binary relationship:

A binary relationship exists when there is an association between two entities.

Ternary relationship:

A ternary relationship exists with there is an association among three entities.

 

Example of an entity-relationship diagram with Er diagram notation:

1. Entity set: Student, examination.
2. Attributes of student: rollno, email, mobile, name.
3. Attributes of Examination: semester, date_time, examid, subject, year.
4. Relationship set: writes.
5. Degree of relationship: binary relationship.
6. Cardinality: many to many relationships(*…*).

 

Also, read Database Replication.