One-to-Many Relationships in a Database

A one-to-many relationship in a database occurs when each record in Table A may have many linked records in Table B, but each record in Table B may have only one corresponding record in Table A.

A one-to-many relationship in a database is the most common relational database design and is at the heart of good design.

Example of a One-to-Many Relationship

Consider the relationship between a teacher and the courses they teach. A teacher can teach multiple classes, but the course would not have the same relationship with the teacher.

Therefore, for each record in a Teachers table, there could be many records in the Courses table. This example illustrates a one-to-many relationship: one teacher to multiple courses.

Why Establishing a One-to-Many Relationship Is Important

To represent a one-to-many relationship, you need at least two tables. Let's see why.

Adherence to First Normal Form Design

Perhaps we created a table in which we want to record the name and courses taught. We might design a Teachers and Courses table like this:

What if Carmen teaches two or more courses? We have two options with this design. We could add it to Carmen's existing record, like this:

However, the design above is inflexible and could result in problems later when you insert, edit, or delete data. It makes it difficult to search for data.

This design also violates the first principle of database normalization, First Normal Form (1NF), which states that each table cell should contain a single, discrete piece of data.

The Second Normal Form Rule

Another design alternative might be to add a second record for Carmen:

This approach adheres to 1NF but is still poor database design because it introduces redundancy and could bloat a large database unnecessarily. More importantly, the data could become inconsistent.

For example, what if Carmen's name changed? Someone working with the data might update her name in one record and fail to update it in the second record.

This design violates the Second Normal Form (2NF) standard, which adheres to 1NF and must also avoid the redundancies of multiple records. The 2NF rule achieves this by separating subsets of data into multiple tables and creating a relationship between them.

How to Design a Database With One-to-Many Relationships

To implement a one-to-many relationship in the Teachers and Courses table, break the tables into two and link them using a foreign key.

Here, we removed the Course column in the Teachers table:

And here is the Courses table. Note that its foreign key, Teacher_ID, links a course to a teacher in the Teachers table:

We have developed a relationship between the Teachers and the Courses table using a foreign key. This arrangement tells us Carmen teaches both Biology and Math and that Jorge teaches English.

We can see how this design avoids any possible redundancies, allows individual teachers to teach multiple courses, and implements a one-to-many relationship.