Database Design: Create a Junction Table to Solve a Many-to-Many Relationship Issue
Many-to-Many Relationship
In many situations, a database will naturally have a many-to-many relationship, such as our example of students having ‘many’ courses and courses having ‘many’ students.
One-to-Many Relationship
In database design, the best relationship that suits our situation is the one-to-many relationship. So, can we change our many-to-many relationship to one-to-many relationship?
Junction Table
The many-to-many relationship can be changed to two one-to-many relationships by adding a junction table.
In the student example, we can add a junction table that registers students on courses. So our junction table is called registration.
Junction Table SQL Create Table
We need to tell SQL to create a relationship, a link, between the tables. The primary key in the initial table, like the student_id in the student table, will now become a foreign key in the junction table, registration.
- add the foreign key first stating the attribute of the foreign key (student_id) in the registration table.
- add references then the other table and the table’s primary key, e.g. student(student_id).
- do the same for the course table and course_code. See the code below:
reg_id INT PRIMARY KEY,
student_id INT,
course_code INT,
FOREIGN KEY (student_id) REFERENCES student(student_id),
FOREIGN KEY (course_code) REFERENCES course(course_code)
);
Junction Table Insert Data
The registration data is entered in the same manner as the other tables although the data must match the other tables, so student_id values must be present in the student table, and the course_code must be in the course table.
In the following examples we have a summary of the insert statement to make it easier to understand, and the full code if you wish to use the full example.
SQL Insert Structure
VALUES
(3001, 652415501, 888342),
…,
(3019, 652415511, 888341)
;
SQL Insert Full Code
INSERT INTO registration (reg_id, student_id, course_code) VALUES
(3001, 652415501, 888342),
(3002, 652415502, 888342),
(3003, 652415503, 881234),
(3004, 652415503, 888342),
(3005, 652415504, 888341),
(3006, 652415504, 888342),
(3007, 652415505, 883123),
(3008, 652415505, 881234),
(3009, 652415506, 884567),
(3010, 652415506, 888341),
(3011, 652415507, 885678),
(3012, 652415508, 886345),
(3013, 652415508, 888342),
(3014, 652415509, 887654),
(3015, 652415509, 888341),
(3016, 652415510, 888120),
(3017, 652415510, 884567),
(3018, 652415511, 888342),
(3019, 652415511, 888341);
Continue to SQL for Multiple Tables
second part of lesson 3: Multiple Table SQL Statements
Continue to Database Design Considerations
final part of lesson 3: Database Design considerations
FAQ
What is the point of a junction table?
A junction table, or bridge table, is used to manage and clarify many-to-many relationships between two tables in a relational database. By introducing a third table containing foreign keys from the related tables, it ensures data integrity and allows you to accurately represent complex relationships. For example, in a library database, a junction table can help associate multiple students with multiple books.
What is a junction table in ERD?
In an Entity-Relationship Diagram (ERD), a junction table appears as a third table between two entities to visualize their many-to-many relationship. It contains two or more foreign keys that reference the primary keys of the related tables. The diagram often represents this relationship with connecting lines that clarify how records are associated.
Are junction tables good practice?
Yes, using junction tables is considered good practice in relational database design. They help maintain the integrity of data, reduce redundancy, and provide flexibility to introduce additional data fields, such as date or feedback fields, to store information specific to the relationship between the two tables.
What is another name for a junction table?
A junction table is also known as a join table, bridge table, or intersection table. Regardless of the name, it serves the same purpose of connecting two related tables to effectively manage and clarify data entry in a relational database.
Understanding Junction Tables
How a Junction Table Works
A junction table contains two foreign keys, each referencing a primary key from one of the related tables. These columns in the junction table help join records from both tables, ensuring that the relationship can be accurately represented.
Example Use Case
Imagine a database for managing employee training sessions. One table lists employees, while another table lists training courses. To track which employees attended which sessions, you create a junction table called “EmployeeTraining” that contains a row for each employee-course pair, with columns for employee ID, course ID, and possibly an additional date field to indicate when the training occurred.
Benefits of Junction Tables
- They help maintain data integrity by avoiding duplicate entries.
- They allow the introduction of composite fields, such as session-specific data.
- They reduce redundancy and streamline data entry.
Design Considerations
When designing a junction table, ensure it:
- Contains at least two foreign keys to relate the tables.
- Supports additional fields if needed, such as date or feedback fields.
- Maintains data constraints to prevent invalid entries.
By using junction tables, database designers can effectively manage complex data relationships and ensure accurate connections between related tables while supporting flexible data organization.