Working with related tables > About relationships > One-to-many relationships
One-to-many relationships
In a one-to-many relationship, one record in a table can be associated with one or more records in another table. For example, each customer can have many sales orders.
A one-to-many relationship looks like this in the relationships graph:
Customers table and orders table with a one to many relationship line between them
In this example the primary key field in the Customers table, Customer ID, is designed to contain unique values. The foreign key field in the Orders table, Customer ID, is designed to allow multiple instances of the same value.
This relationship returns related records when the value in the Customer ID field in the Orders table is the same as the value in the Customer ID field in the Customers table.
Records in customers and orders tables showing result of one to many relationship
Because relationships work both ways, there are also many-to-one relationships.
Related topics 
Planning a relational database
Working with the relationships graph
Creating and changing relationships