In the given scenario, a data designer can identify many-to-many relationships based on the following Entity-Relationship Diagram (ERD) modeling constraints:
- A student may enroll in many classes (1:M);
- A class may have many enrolled students (1:N).
To identify the many-to-many relationship based on these constraints, follow these steps:
- Understand the entities and constraints: First, identify the entities involved in the relationship: 'Student' and 'Class'. Next, analyze the constraints, which state that a student can enroll in multiple classes, and a class can have multiple enrolled students.
- Determine relationship cardinality: In this case, the relationship between 'Student' and 'Class' entities is many-to-many (M:N) because multiple instances of each entity are related to multiple instances of the other entity. A many-to-many relationship occurs when instances of one entity can be associated with multiple instances of another entity, and vice versa.
- Introduce an associative entity: Many-to-many relationships cannot be directly implemented in relational databases, so a data designer must introduce an associative entity (also known as a junction table or linking table) to break down the many-to-many relationship into two one-to-many relationships. The associative entity holds the primary keys of both related entities, creating a composite primary key.
- Create the associative entity: In this scenario, create an associative entity called 'Enrollment' to represent the relationship between 'Student' and 'Class'. The 'Enrollment' entity should have the primary keys of both the 'Student' and 'Class' entities as its attributes (e.g., 'Student_ID' and 'Class_ID').
- Establish one-to-many relationships: Now, create two one-to-many relatonships:
a. Between 'Student' and 'Enrollment': A student may have multiple enrollments, but each enrollment is associated with only one student (1:M).
b. Between 'Class' and 'Enrollment': A class may have multiple enrollments, but each enrollment is associated with only one class (1:N).
- Use notation to represent relationships: In the ERD, represent the one-to-many relationships using lines connecting the related entities. Add a crow's foot symbol at the end of the associative entity ('Enrollment') and a straight line or a single bar at the end of the parent entities ('Student' and 'Class') to signify the one-to-many relationships.
- Validate the relationships: Review the resulting ERD to ensure it accurately reflects the given constraints and effectively represents the many-to-many relationship between 'Student' and 'Class' entities. Consult with stakeholders and subject matter experts to confirm that the ERD meets the business requirements.
By following these steps, a data designer can identify the many-to-many relationship between 'Student' and 'Class' entities based on the given constraints, and create an ERD that accurately represents the data structure in a format suitable for implementation in a relational database.
Many-to-many relationships are also very common. There is, for example, a many-to-many relationship between an order placed by an DistributedNetworks customer and the merchandise items carried by the store. An order can contain multiple items, and each item can appear on more than one order. The same is true of the orders placed with distributors. An order can contain multiple items and each item can appear on more than one order.
A many-to-many relationship
exists between entities A and B if for two instances of those entities (Ai and Bi)
- Ai can be related to zero, one, or more instances of entity B and
- Bi can be related to zero, one, or more instances of entity A.
Many-to-many relationships present two major problems to the design of a database.
These issues and the way in which we solve them are discussed later. The next lesson explains the problems with many-to-many relationships.