Attributes of Relationship Types

 

Attributes of Relationship Types

In the ER model, relationship types—just like entity types—can have attributes. These attributes describe properties that belong to the association itself, rather than to any single participating entity.

Why Do Relationships Need Attributes?

Some facts in the miniworld depend on how two (or more) entities are related, not on the entities individually.

Examples

  • The number of hours an employee works on a project

  • The date an employee started managing a department

These values cannot be correctly stored as attributes of only one entity in all cases.

Examples from the COMPANY Database

WORKS_ON Relationship

  • Relationship: EMPLOYEE ↔ PROJECT (M:N)

  • Attribute: Hours

➡️ The number of hours depends on the employee–project pair, not on either entity alone.

MANAGES Relationship

  • Relationship: EMPLOYEE ↔ DEPARTMENT (1:1)

  • Attribute: Start_date

➡️ The start date indicates when the management relationship began, not a property of just the employee or the department.

Migrating Relationship Attributes

Whether a relationship attribute stays with the relationship or moves to an entity depends on the cardinality of the relationship.

1:1 Relationships

  • Each entity participates in at most one relationship instance

  • Relationship attributes can be moved to either participating entity

Example:

  • MANAGES (1:1)

  • Start_date can be stored in:

    • EMPLOYEE (manager), or

    • DEPARTMENT

✔ Both choices are valid
✔ Decision is left to the schema designer

1:N Relationships

  • Relationship attributes can be moved only to the N-side entity

Example:

  • WORKS_FOR (1:N)

  • Attribute: Start_date (employee joining date)

✔ Stored in EMPLOYEE, not in DEPARTMENT
✔ Because each employee participates in only one WORKS_FOR instance

M:N Relationships

  • Relationship attributes cannot be moved to either entity

  • They must remain as relationship attributes

Example:

  • WORKS_ON (M:N)

  • Attribute: Hours

✔ Hours depends on both employee and project
✔ Must be stored in the relationship


Summary Table

Relationship Type    Where Relationship Attributes Can Go
1:1    Either entity or the relationship
1:N    Only on the N-side entity
M:N    Must remain in the relationship

Key Takeaways

✔ Relationship types can have attributes
✔ These attributes describe associations, not entities
✔ Migration depends on cardinality constraints
✔ M:N relationship attributes must stay in the relationship
✔ Design choices in 1:1 and 1:N are often conceptual and subjective

Comments

Post a Comment

Popular posts from this blog

Database Management Systems DBMS PCCST402 Semester 4 KTU CS 2024 Scheme

 About Me Database Management Systems DBMS PCCST402 Scheme and Syllabus Module-1 Introduction to DBMS Introduction to DBMS Example  Database Database System Vs Traditional File Processing Database System Concepts and Architecture Data Models, Schemas and Instances   Three schema architecture and Data Independence Database Languages and Interfaces Centralized and Client Server Architectures in DBMS Classification of Database Management Systems Conceptual Data Modeling and Database Design Conceptual Modeling and the Role of the ER Model in Database Design Entities and Attributes  Company Database Application - Example Entity Types, Entity Sets, Keys, and Value Sets in the ER Model Initial Conceptual Design of the COMPANY Database Relationship Types in the ER Model Relationship Degree, Role Names, and Recursive Relationships Constraints on Binary Relationship Types Attributes of Relationship Types Weak Entity Types Summary of the Notations Used in ER Diagram , Naming Co...
Read more

Database Management Systems DBMS PCCST402 Scheme and Syllabus

  DATABASE MANAGEMENT  SYSTEMS (Common to  CS/CD/CA/CR/AD/AI/CB/CN/CC/CU/CI/CG)   Course  Code PCCST402 CIE Marks 40 Teaching  Hours/Week (L:T:P: R) 3:1:0:0 ESE Marks 60 Credits 4 Exam Hours 2Hrs.30 Min. Prerequisites(if  any) PCCST303 Course Type Theory Course Objectives:   1.        Equip the students with a comprehensive understanding of fundamental DBMS concepts as well as the principles and applications of NoSQL databases 2.        Enable students to design,implement,and manage both relational and NoSQL databases SYLLABUS   Module No. Syllabus  Description Contact Hours         1 Introduction to Databases...
Read more

Introduction to Database Management System -DBMS

Image
In today’s digital world, almost every application—from mobile banking and e-commerce to university portals and hospital systems—depends on the efficient storage, organization, and retrieval of data. As the amount of information generated by individuals and organizations continues to grow, managing this data in a structured and meaningful way becomes essential. This is where database systems play a central role. A database can be thought of as a well-organized collection of related data, designed so that it can be easily accessed, managed, and updated. But the database itself is only one component. To manage it effectively, we rely on a specialized software package known as a Database Management System (DBMS) . A DBMS provides a systematic and automated way to store large volumes of data, enforce rules for consistency and correctness, protect data from unauthorized access, and allow multiple users to work with the data simultaneously without conflicts. A database typically has three...
Read more