Working with related tables > About relationships > One-to-one relationships
One-to-one relationships
In a one-to-one relationship, one record in a table is associated with one and only one record in another table. For example, in a school database, each student has only one student ID, and each student ID is assigned to only one person.
A one-to-one relationship looks like this in the relationships graph:
Students table and contact info table with a one to one relationship line between them
In this example, the key field in each table, Student ID, is designed to contain unique values. In the Students table, the Student ID field is the primary key; in the Contact Info table, the Student ID field is a foreign key.
This relationship returns related records when the value in the Student ID field in the Contact Info table is the same as the Student ID field in the Students table.
Records in students and contact info tables showing result of one to one relationship
Related topics 
Planning a relational database
Working with the relationships graph
Creating and changing relationships