One-to-One

ELI5 — The Vibe Check

One-to-One is a relationship where one row in Table A corresponds to exactly one row in Table B. Like a user and their profile — one user, one profile. You could put it all in one table but sometimes you split for organization or performance.

Real Talk

A one-to-one relationship between two entities means each record in Table A is related to exactly one record in Table B and vice versa. Implemented with a foreign key and UNIQUE constraint. Often used to split a table for optional data, security, or performance (e.g., users and user_settings).

Show Me The Code

CREATE TABLE user_profiles (
  user_id INT PRIMARY KEY REFERENCES users(id),
  bio TEXT,
  avatar_url TEXT
);

When You'll Hear This

"The user and user_profile tables have a one-to-one relationship." / "One-to-one is often better merged into one table unless there's a reason to split."