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A **one-to-one (1:1)** relationship is a type of table relationship where **each record in one table corresponds to at most one record in another**.

This kind of relationship is created rarely, but it's useful when you need to **split one entity's data into two logical blocks**.

### Example

Say we have a `users` table and a `passports` table:

```sql
CREATE TABLE users (
  id INT PRIMARY KEY,
  name VARCHAR(50)
);

CREATE TABLE passports (
  id INT PRIMARY KEY,
  user_id INT UNIQUE,
  passport_number VARCHAR(20),
  FOREIGN KEY (user_id) REFERENCES users(id)
);
```

Here:

- `users.id` is the primary key of the users table,
- `passports.user_id` is a foreign key that **references** `users.id` and has a `UNIQUE` constraint.

Thanks to `UNIQUE`, one user can have **only one passport**,
and one passport has **only one owner**.

### How it works

| users table | passports table |
|---|---|
| id = 1, name = 'Anna' | user_id = 1, passport_number = 'AB123456' |
| id = 2, name = 'Ivan' | user_id = 2, passport_number = 'CD987654' |

Each `users` row can be linked to **only one** `passports` row.

### When 1:1 is used

1. **To separate private data.**
For example, personal data (passport, tax ID) gets moved into a separate table with restricted access.
2. **For optimization.**
To avoid too many fields in one table, splitting by meaning (e.g. `users` and `user_profiles`).
3. **For rare extensions.**
When extra information applies to only some records. For example, if only some employees have a company car, the `cars` table would relate to `employees` as 1:1.

### Example query

```sql
SELECT users.name, passports.passport_number
FROM users
JOIN passports ON users.id = passports.user_id;
```

We get the user's name and their passport, one-to-one.

**Summary:**
A **1:1** relationship is a strict pair, "one record <-> one record."
It guarantees a unique relation between tables,
and is used for separation, security, and structuring data.

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