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
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- 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.
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- 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:
- Entity set: Student, examination.
- Attributes of student: rollno, email, mobile, name.
- Attributes of Examination: semester, date_time, examid, subject, year.
- Relationship set: writes.
- Degree of relationship: binary relationship.
- Cardinality: many to many relationships(*…*).
Also, read Database Replication.