Question

Describe the entity-relationship model. How are entities, relationships, and attributes represented in this model?What is a composite entity? Describe the approach to diagrams that use a crow's foot. Describe how you wouldrepresent cardinality in an E-R diagram.

Answer 1

ER Model is an abbreviation for Entity-Relationship Model. A mere full-form of the term explains to us that the ER Model is some kind of a diagram that is going to show us the Relationships established between Entities. An Entity-Relationship Model is resourceful in designing the logical view of any system from a Designer's or a Modeler's perspective. Usually, this Model diagram-illustration is used in systems where the Object-Oriented Programming via the OOPS Methodology is utilized.

What kind of Entities? Entities in Computer systems represent the Real-world objects and those objects have certain relationships among them which are represented by varied kinds of Relationships in the Computer system. Entities are objects with any kind of physical existence or conceptual existence. Example: person, vehicle, estate, enterprise, tasks or academic program. Entity Type is the object and Entity Set is the collection-group of all entities Example is as illustrated in the diagram below.

A. Attribute: The characteristics of the Entities that describe the properties of the specific entity are called as Attributes of the Entity. Example: Employee_ID, Emp_Name, DOB, Age, Address, Mobile_No, Emp_Salary, etc. are the attributes that describe the Entity Type Employee. The Attribute is represented by a Horizontal Oval in any ER diagram.

Types of Attributes as follows:

1. Key Attribute: Uniquely identifies each and every entity in the list of entity sets. Example: Roll_No is unique to every Student. The key attribute is represented by an oval with underlying lines.

2. Composite Attribute: Composite attribute is always composed of many different attributes. Example: Attribute Address of Entity type Student comprises of Street, City, State, and Country. Composite Attribute is represented by an oval comprising of many other ovals.

3. Multivalued Attribute: Consists of more than one value for a specific entity. Example: Phone_No (can be more than one for a given student). Any Multivalued Attribute is represented by a double oval.

4. Derived Attribute: Derived Attributes are derived from other attributes Example: Age (can be derived from Date-Of-Birth). Derived Attribute is represented by a dashed oval.

The complete entity type Student with its attributes can be represented as:

Relationship Type: The kind of association among the same or different kinds of entities is the Entity-Relationship Type. Example: A relationship type ‘Enrolled in’ exists between entities Student and Course. A Relationship Type, in the ER model, is represented as a Diamond that is connecting the entities with lines.

Relationship Set: A Set of similar types of relationships. Example: S, E, and C are different Entity Sets (comprising various Entities within) where S1 is enrolled in C2, S2 is enrolled in C1 and S3 is enrolled in C3.

Relationship Set Degree: Number/amount of Entity Sets participating in a relationship set.

1. Unary Relationship: The relationship is called a Unary relationship when Only ONE entity set is participating in a relation. Example: One Person is Married to Only One other Person.

   

2. Binary Relationship: The relationship is called a Binary relationship if there are Two Entities' set participating in a relation. Example: A Student is enrolled in a Course.

   

3. n-ary Relationship: n number of Entities' set participating in a relation is called an n-ary relationship.

Cardinality in an ER Diagram: Cardinality of any relationship in the ER Model can be defined as the number of times relationship set's entity from an entity set participates.

Types of Cardinality:

1. One-to-one: When each entity in each entity set can take part only once in the relationship, the cardinality is one-to-one. Example: One Male can Marry to One Female and One Female can Marry One Male. So the relationship will be one-to-one.

   

   Using Entities' Sets, One-to-One Relationship is represented as:
   (M==Male; F==Female; R==Relationship)
   

2. Many-to-one: When entities in one entity set can take part only once in the relationship set and entities in other entity sets can take part more than once in the relationship set, their Cardinality is many-to-one. Example: One Student takes Only One Course but One Course can be taken by Many Students. The Cardinality here is n-to-1. This means, for 1 Course there can be n number of Students but for One Student, there will be Only One Course.

   
   Using Entities' Sets, it is represented as:
   (S==Student; C==Course; R==Relationship)
   

   In this case, each student is taking only 1 course but 1 course has been taken by many students.

3. Many-to-many: When entities in all entity sets can take part more than once in the relationship, the Cardinality is many-to-many. Example: One Student can take more than One Course and one course can be taken by Many Students. So, the relationship will be many-to-many.
   

   Using Entities' Sets, it is represented as:
   (S==Student; C==Course; R==Relationship)
   

In this example, student S1 is enrolled in C1 and C3 and Course C3 is enrolled by S1, S3, and S4. So it is many-to-many relationships.

Crow's Foot: (Due to Shortage of Time, I am not including a detailed explanation here)

Crow's Foot notation facilitates the relationships to not have any Attribute. When the Crow's Foot notation is implemented, the relationships are elevated or promoted to entities and entity sets. Example: if it is necessary to record where and when any Employee worked and performed any specific task excellently, a new entity "performance" is created and attributed reflecting with the timestamp as well as the location, company_name, and the relationship-association of the employee to the specific job-task becomes an indirect association relationship via the performance (employee-works-performance, performance-facilitates-job).

The symbols used to represent Cardinality along with its various Relationships and Cardinalities as follows: