What is the purpose of a one-to-one relationship between tables in a database?

A one-to-one relationship between tables in a database is used to connect two tables so that each record in the first table corresponds to exactly one record in the second, and vice versa. This is useful when you need to split data into logical parts or when one of the tables contains additional optional data.

For example, an application might have a Users table with basic user information and a UserProfiles table with additional data that is not always mandatory. A one-to-one relationship allows storing this data separately, maintaining integrity and simplifying the structure.

In Go, when working with ORM (e.g., GORM), a one-to-one relationship can be described as follows:

 type User struct {
     ID       uint
     Name     string
     Profile  UserProfile
 }
 
 type UserProfile struct {
     ID     uint
     UserID uint
     Bio    string
 }

Here, UserProfile is linked to User through the UserID field, and each user has exactly one profile.