5-3 Mapping ER Diagrams to Relational Schemas Explained
Key Concepts
- Entity
- Attribute
- Relationship
- Primary Key
- Foreign Key
Entity
An entity is a distinct object or concept in the real world that is represented in a database. Each entity has attributes that describe its properties. In a relational schema, an entity is typically represented as a table.
Example: In a university database, "Student" could be an entity with attributes like StudentID, Name, and Major.
Analogies: Think of an entity as a person in a class roster or a product in an inventory list.
Attribute
An attribute is a property or characteristic of an entity. Each attribute has a name and a data type. In a relational schema, attributes are represented as columns in a table.
Example: For the "Student" entity, attributes could include StudentID (integer), Name (varchar), and Major (varchar).
Analogies: Think of attributes as the details listed on a student ID card or the specifications of a product in a catalog.
Relationship
A relationship defines how entities interact with each other. Relationships can be one-to-one, one-to-many, or many-to-many. In a relational schema, relationships are represented using foreign keys.
Example: A "Student" entity might have a one-to-many relationship with a "Course" entity, where each student can enroll in multiple courses.
Analogies: Think of a relationship as a connection between a student and their enrolled courses, or between a product and its suppliers.
Primary Key
A primary key is a unique identifier for each record in a table. It ensures that each row in the table is distinct. A primary key cannot be null and must be unique.
Example: For the "Student" table, StudentID could be the primary key.
Analogies: Think of a primary key as a student's unique ID number or a product's unique barcode.
Foreign Key
A foreign key is a field in one table that refers to the primary key in another table. It establishes a link between the data in two tables and enforces referential integrity.
Example: In a "StudentCourse" table that links students to courses, StudentID would be a foreign key referring to the "Student" table, and CourseID would be a foreign key referring to the "Course" table.
Analogies: Think of a foreign key as a reference number that connects a student's record to their course enrollment records.
Conclusion
Mapping ER diagrams to relational schemas is a crucial step in database design. By understanding entities, attributes, relationships, primary keys, and foreign keys, a Database Specialist can effectively translate conceptual models into practical, functional databases. This process ensures data integrity and efficient data management.