One-to-Many Relationships in a Database

A One-to-Many Relationship Helps Achieve Data Integrity

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. 

Consider the relationship between a teacher and the courses they teach. A teacher can teach multiple courses, 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 is 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.

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

Teachers and Courses
Teacher_IDTeacher_NameCourse
Teacher_001CarmenBiology
Teacher_002VeronicaMath
Teacher_003JorgeEnglish

 

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

Teachers and Courses
Teacher_IDTeacher_NameCourse
Teacher_001CarmenBiology, Math
Teacher_002VeronicaMath
Teacher_003JorgeEnglish

 

The design above, however, is inflexible and could result in problems later when trying to insert, edit or delete data.

It makes it difficult to search for data. This design 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.

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

Teachers and Courses
Teacher_IDTeacher_NameCourse
Teacher_001CarmenBiology
Teacher_001CarmenMath
Teacher_002VeronicaMath
Teacher_003JorgeEnglish

 

This adheres to 1NF but is still poor database design because it introduces redundancy and could bloat a very 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 Second Normal Form (2NF), which adheres to 1NF and must also avoid the redundancies of multiple records 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, we break the tables into two, and link them using a foreign key.

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

Teachers
Teacher_IDTeacher_Name
Teacher_001Carmen
Teacher_002Veronica
Teacher_003Jorge

 

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

Courses
Course_IDCourse_NameTeacher_ID
Course_001BiologyTeacher_001
Course_002MathTeacher_001
Course_003EnglishTeacher_003

 

We have developed a relationship between the Teachers and the Courses table using a foreign key.

This tells us that both Biology and Math are taught by Carmen 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.

Databases can also implement a one-to-one relationship and a many-to-many relationship.