SQL - One to One Relationship

In a database, a one-to-one relationship is a type of relationship between two tables where each record in one table is associated with exactly one record in the other table, and vice versa. This relationship is based on a shared key between the two tables, typically a primary key in one table and a foreign key in the other. Here's an example to illustrate a one-to-one relationship:

Consider a database for a company that stores employee information. We have two tables: "Employees" and "EmployeeDetails."

Table: Employees

In this example, the "Employees" table stores basic information about employees, including their EmployeeID, FirstName, and LastName. The "EmployeeDetails" table stores additional details for each employee, such as their address and phone number.

To establish a one-to-one relationship, the EmployeeID column in the "Employees" table serves as the primary key, and it is also included in the "EmployeeDetails" table as a foreign key. Each EmployeeID in the "Employees" table corresponds to a unique EmployeeID in the "EmployeeDetails" table, and vice versa.

This one-to-one relationship ensures that each employee in the "Employees" table has only one associated record in the "EmployeeDetails" table, and each record in the "EmployeeDetails" table is linked to only one employee in the "Employees" table.

This type of relationship is useful when we want to separate certain attributes into a separate table to avoid data duplication or when we have optional data that doesn't apply to all employees. In this example, the additional details in the "EmployeeDetails" table are not mandatory for all employees, and the one-to-one relationship allows flexibility in storing and managing this optional information.

Overall, a one-to-one relationship establishes a direct and unique association between records in two tables, where each record in one table corresponds to exactly one record in the other table.