Example 1: Transforming ER Diagrams to a Relational Schema

Summary

In this tutorial, the speaker revisits the process of transforming ER diagrams into relational schemas, addressing user requests for further explanation. The example demonstrates the conversion of a diagram involving four entities—Car, Customer, Invoice, and Employee—each with various attributes and relationships. The speaker details the handling of composite and multivalued attributes, as well as the placement of foreign keys based on one-to-many relationships. The video walks through how to properly structure the relational schema by defining tables, primary keys, and foreign keys, ensuring clarity for database design.

Takeaways

• 😀 Composite attributes should only include their component parts when transforming into relational schemas.
• 😀 In a one-to-many binary relationship, the foreign key is placed on the 'many' side.
• 😀 Multivalued attributes require the creation of a new relation linking back to the original entity.
• 😀 The foreign key in a one-to-many relationship is based on the primary key of the 'one' side.
• 😀 A primary key uniquely identifies each entity and is essential in relational schema creation.
• 😀 For multivalued attributes, the new relation should use the entity’s primary key and the multivalued attribute as a composite primary key.
• 😀 Car entities involved in relationships will have foreign keys referencing both customer and employee entities.
• 😀 No foreign keys are added to an entity on the 'one' side of a one-to-many relationship.
• 😀 Employee entities with multivalued attributes, such as qualifications, require a new relation to handle them.
• 😀 Relationships like invoice and customer, or invoice and employee, add foreign keys to the 'many' side based on the entity's primary keys.

Q & A

What is the purpose of transforming an ER diagram into a relational schema?

The purpose of transforming an ER diagram into a relational schema is to represent the entities, their attributes, and the relationships between them in a format that can be implemented in a relational database. This process helps in structuring the data logically for easier storage, retrieval, and manipulation.

Why do we not include composite attributes like 'Model' in the relational schema?

In relational schema transformation, composite attributes are not included directly. Instead, only the component parts of a composite attribute are included. For example, the 'Model' attribute is broken down into its component parts like 'Model Number', 'Color', and 'Year'.

What does it mean when a relationship is described as 'one-to-many'?

A 'one-to-many' relationship means that one entity is associated with multiple instances of another entity. For example, one customer can own multiple cars, but each car is associated with only one customer.

How is the foreign key placed in a 'one-to-many' relationship?

In a 'one-to-many' relationship, the foreign key is placed on the 'many' side of the relationship. This means that the entity on the 'many' side will have a foreign key referring to the primary key of the entity on the 'one' side.

Why is 'Customer_ID' added as a foreign key in the 'Car' entity?

Since the relationship between 'Car' and 'Customer' is 'one-to-many' and the 'Customer' is on the 'one' side, 'Customer_ID' is added as a foreign key in the 'Car' entity to establish the link between the customer and the car.

What should be done when an attribute is a multivalued attribute?

When an attribute is multivalued, a new relation (or table) should be created to represent that multivalued attribute. The new relation will include the primary key of the original entity as a foreign key, along with the multivalued attribute itself.

How is the multivalued attribute 'Qualification' handled in the 'Employee' entity?

The multivalued attribute 'Qualification' in the 'Employee' entity is handled by creating a new relation called 'Employee_Qualification'. This new table will have a composite primary key consisting of 'Employee_ID' and 'Qualification'. The 'Employee_ID' acts as a foreign key referencing the 'Employee' entity.

Why are there no foreign keys added in the 'Customer' entity despite having relationships with other entities?

No foreign keys are added in the 'Customer' entity because in both relationships it participates in (with 'Car' and 'Invoice'), it is on the 'one' side. According to relational schema rules, foreign keys are added to the 'many' side of relationships.

How are simple attributes handled in the relational schema?

Simple attributes are directly included as columns in the corresponding table in the relational schema. For example, the 'Customer' entity's simple attributes like 'Customer_ID', 'Name', and 'Address' are directly copied to the 'Customer' table.

What happens when an entity like 'Invoice' is involved in multiple relationships?

When an entity like 'Invoice' is involved in multiple relationships, foreign keys are added for each of the relationships it is involved in. For example, 'Invoice' will have 'Customer_ID' and 'Employee_ID' as foreign keys linking it to 'Customer' and 'Employee' respectively, because it is on the 'many' side in both relationships.