E-R Diagram

ER-Diagram is a visual representation of data that describes how data is related to each other

Components of E-R Diagram

1. Entity:  Entity refers to a real world object that might depict a table in an SQL Server database. Databases are often constructed in the basis of an Entity-relationship diagram.

2.Weak Entity : In a relational database, weak entity  is a entity that cannot be uniquely identified by its attributes, it should take the help  of foreign key in conjunction with its attributes to create a primary key.

3. Attribute: It defines the properties of the table.or example, Name, Age, Address etc can be attributes of a Student.

4. Key Attribute: It represents  the primary key in a particular entity/element

For example, id in the student entity type.

5. Non-Key Attribute: These are the attributes other than candidate key  in a given table.For example in Name, First Name is non-Key Attribute which does not have the main characteristics of the entity.

6. Composite Attribute: In this type, all the attributes are arranged in a tree-structured way and every node is connected to its attribute.

Relationship:

It describes the bond  between the entities.

There are generally three types of relationships:

1. Binary relationship

2. Recursive Relationship

3. Ternary Relationship

BINARY RELATIONSHIP

Binary Relationship means relation between two Entities. This is further divided into three types.

1. One to One : This type of relationship is rarely seen in real world.

2. One to Many : It reflects business rule that one entity is associated with many number of same entity. The example for this relation might sound a little weird, but this means that one student can enroll to many courses, but one course will have one Student.

Many to One : It reflects business rule that many entities can be associated with just one entity. For example, Student enrolls for only one Course but a Course can have many Students.

RECURSIVE RELATIONSHIP

When an Entity is related with itself it is known as Recursive Relationship.

TERNARY RELATIONSHIP

Relationship of degree three is called Ternary relationship.