E-R-Model describe data and identifies relationship between them.E-R model is represented by E-R Diagram.E-R- model is explained along with their represented in E-R Diagram.
Relationship-: A Relationship is an association between entities.
A relationship is represented as a diamond between two entities types. It has a label that explains the relationship usually the convention is to read the E-R diagram from top to bottom and from left to right so the relationship name is so chosen as to make sense.When read from left to right the relationship above is read as student enrols in course.
Unary Relationship-: It is represented as a diamond which connect one entity to itself as a loop. the relationship below means some instance of employee manage other instance of Employee.
Ternary Relationship-: When a relationship between three entity its called ternary relationship.
Relationship Participation – It is two types
- Partial -: All instances of the entity type employee don’t participated in the relationship head of i.e. every employee doesn’t head a department. So employee entity type is said to partially participated in the relationship but every department would be headed by some employee.
- Total-: All instances of the entity type department participated in this relationship, so we say that it is total participated from the department side.
Attribute of relationship-: These attributes best describe the relationship rather then any individual entity.
Weak Entity-: Entity that depends on other entity for it’s existence and doesn’t have key attribute of its own is called weak entity.
The dependent entity is represented by a double lined rectangle and the identifying relationship by a double lined diamond identifying relationship is the one which relates the weak entity with the strong entity on which it depends.
Types of Relationships Cardinality -: An entity may be associated with one none or many occurrences of another entity in E-R Diagrams. The types of relationships are represented as follow..
- One and only one
- One or More
- Zero or More
- Zero Or One