How to implement a one-to-one relationship in a database?

A one-to-one relationship in a database is implemented using a foreign key with a unique constraint.

Example:

There are two tables: User and UserProfile. Each user has exactly one profile.

CREATE TABLE User (
  id SERIAL PRIMARY KEY,
  username VARCHAR(50) NOT NULL
);

CREATE TABLE UserProfile (
  id SERIAL PRIMARY KEY,
  user_id INT UNIQUE REFERENCES User(id),
  bio TEXT
);

Here, user_id in UserProfile is a foreign key to User.id with a UNIQUE constraint, ensuring that each user has at most one profile.

In Java, using JPA/Hibernate, this can be described as:

@Entity
public class User {
    @Id
    private Long id;

    @OneToOne(mappedBy = "user", cascade = CascadeType.ALL)
    private UserProfile profile;
}

@Entity
public class UserProfile {
    @Id
    private Long id;

    @OneToOne
    @JoinColumn(name = "user_id", unique = true)
    private User user;
}

Thus, the one-to-one relationship is enforced at the database and ORM level.