Definition - What does One-to-One Relationship mean?
A one-to-one relationship in a relational database occurs when one parent record or field has either zero or one child record only. These relationships are the easiest to represent in databases because both the parent and child records may be in the same table.
Techopedia explains One-to-One Relationship
As an example, in the CUSTOMER_MASTER table in a bank’s database, each customer is represented by a unique CUSTOMER_ID, which is also the table’s primary key. Each customer may also have a government issued social security card, which contains a unique social security number. Therefore, each customer must have a single customer ID in the bank’s database. If a customer does have one, then there can only be one social security number per customer.
The parent field (CUSTOMER_ID) has a one-to-one relationship with the social security field. For such relationships, it is best to put them into one table for ease of reference. In this case, the social security number should simply be an additional column in the CUSTOMER_MASTER table.