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Lesson 2 Entity Relationships
ObjectiveDefine the concept of an Entity Relationship

Associations between Entities

How are entity relationships used in data modeling?
Entity Relationship (ER) modeling is a fundamental technique used in the process of data modeling to systematically represent the structure and organization of information in a conceptual, logical, and eventually physical database design. ER modeling employs a set of graphical notations and symbols to define and visualize the relationships between entities in a system, providing a clear and concise view of the data and its underlying connections.
Entities are the main building blocks of an ER model, representing real-world objects or concepts that hold significance within a specific domain. Examples of entities include customers, products, or employees. Entities are composed of attributes, which define the specific characteristics or properties of an entity, such as a customer's name, address, and phone number.
Relationships, on the other hand, are associations between entities that help to describe how they are interconnected. For example, a relationship might exist between a customer entity and an order entity to indicate that a customer can place multiple orders. Relationships can be further classified into different types, such as one-to-one, one-to-many, or many-to-many, based on the cardinality between the participating entities.
ER modeling provides several benefits in data modeling, including:
  1. Clarity and simplicity: ER diagrams offer a visual representation of the data structure, making it easier for stakeholders to comprehend and validate the design. This reduces the chances of misinterpretation and ensures that the final database implementation aligns with the intended model.
  2. Systematic approach: ER modeling follows a structured methodology, promoting consistency and coherence in the design process. This systematic approach allows for better organization and management of complex data systems.
  3. Abstraction: ER models enable abstraction, allowing designers to focus on the high-level relationships between entities and the essential properties of the data. This makes it easier to identify and address potential issues or inconsistencies early in the design process.
  4. Reusability and scalability: ER models facilitate the reuse of design components and promote modularity. This makes it easier to maintain, update, and scale the data model as the system evolves over time.
  5. Effective communication: By providing a standardized visual representation of the data structure, ER models facilitate communication among various stakeholders, such as developers, database administrators, and end-users. This helps to ensure a shared understanding of the system and streamline the development process.

Entity Relationship modeling is an essential tool in data modeling, allowing designers to create clear and concise representations of complex data structures. By visually mapping out the relationships between entities, ER models enable a better understanding of the system, improved communication among stakeholders, and the efficient development and management of the resulting database.

Define Concept of Entity Relationship

Entity relationships are associations between entities, which you have come to recognize as links between tables. Typically, a relationship is indicated by a verb connecting two entities. Consider the following two related entities with several attributes listed for each (key attributes are indicated with asterisks).

ERD Diagram consisting of CUSTOMER, ORDER, LINE ITEM, DISTRIBUTOR, and CD table
Two related entities: Employees are assigned to projects OR Projects are assigned to employees.

A general statement about the relationship between the EMPLOYEE and PROJECT entities may be put this way:
  1. Employees are assigned to projects (or)
  2. Projects are assigned to employees.

Although these statements denote an association between the two entities, we know nothing about how the entities are formally related to one another. Until we know considerably more, we cannot convert them into an ER diagram or, ultimately, into linked tables.
For example, one important thing we do not yet know is if the EMPLOYEE entity should insert its key attribute (EmpID) into the PROJECT entity to establish the relationship, or if the PROJECT entity should insert its key attribute (ProjID) into the EMPLOYEE entity. We also do not know if every employee must be assigned to a project, or if deleting an employee record from an Employees table will have an adverse effect on records in a linked Projects table.
Determining how entities are related requires careful interpretation of the business rules uncovered during Requirements Analysis (and the application of common-sense rules). Rules tell the database designer the type of relationship that exists between entities, and the type of participation entities enjoy in relationships.

While it is true that the entity-relationship model of a database is independent of the formal data model used to express the structure of the data to a DBMS, we often make decisions on how to model the data based on the requirement of the formal data model we will be using. Removing multivalued attributes is one such case and you will also see an example of this when we deal with many-to-many relationships between entities.

Entity Relationships - Exercise

Before moving on to the next lesson, click the Exercise link below to check your understanding of entity relationships.
Entity Relationships - Exercise
The next lesson briefly describes the types of relationships that can exist between entities.

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