In this section, we will explore how to transform Entity-Relationship (ER) diagrams into relational schemas. This process is crucial for designing a database that accurately represents the data and its relationships. We will cover the following steps:

1. Identifying Entities and Attributes
2. Mapping Entities to Tables
3. Mapping Relationships
4. Handling Special Cases

Entities represent real-world objects or concepts. In an ER diagram, entities are typically represented by rectangles.

Attributes are properties or characteristics of entities. They are usually represented by ovals connected to their respective entities.

Example: Consider an ER diagram with two entities: Student and Course.

• Student: StudentID, Name, Email
• Course: CourseID, Title, Credits

Each entity in the ER diagram is mapped to a table in the relational schema. The attributes of the entity become the columns of the table.

Example:

In a one-to-one relationship, each instance of an entity is associated with a single instance of another entity. This can be implemented by adding a foreign key to either of the tables.

Example: If each Student has one Profile, you can add a ProfileID foreign key to the Student table.

In a one-to-many relationship, one instance of an entity is associated with multiple instances of another entity. This is implemented by adding a foreign key to the table representing the "many" side of the relationship.

Example: If a Course can have multiple Students, add a CourseID foreign key to the Student table.

In a many-to-many relationship, multiple instances of an entity are associated with multiple instances of another entity. This requires creating a junction table that includes foreign keys referencing the primary keys of both entities.

Example: If students can enroll in multiple courses, create an Enrollment table.

Composite attributes can be broken down into simpler attributes. For example, a FullName attribute can be divided into FirstName and LastName.

Multi-valued attributes should be represented by a separate table. For example, if a Student can have multiple PhoneNumbers, create a PhoneNumbers table.

Weak entities depend on another entity for their existence and do not have a primary key of their own. They are represented by a table that includes a foreign key referencing the related entity's primary key.

Example: If Order is a weak entity dependent on Customer, the Order table will include a CustomerID foreign key.

Let's consider a more comprehensive example involving Students, Courses, and Enrollments.

• Entities: Student, Course
• Attributes:
  • Student: StudentID, Name, Email
  • Course: CourseID, Title, Credits
• Relationships:
  • Student enrolls in Course (Many-to-Many)

Given the following ER diagram, transform it into a relational schema.

Entities:

• Author: AuthorID, Name
• Book: BookID, Title, PublicationYear

Relationships:

• Author writes Book (One-to-Many)

Given the following ER diagram, transform it into a relational schema.

Entities:

• Employee: EmployeeID, FullName, PhoneNumbers

Attributes:

• FullName: FirstName, LastName
• PhoneNumbers: Multiple values

Transforming ER diagrams into relational schemas is a fundamental skill in database design. By following the steps outlined in this section, you can ensure that your database accurately represents the data and its relationships. Practice with different ER diagrams to become proficient in this process.