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Relationships between tables are **logical connections between data** in different tables that let SQL "understand" how to join rows in queries.

They're built through **keys**, special fields that link tables together.

### 1. Primary key (PRIMARY KEY)

This is **a unique identifier** for a row in a table.
Each record has its own non-repeating value.
Example:

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

The `id` field is the user's unique identifier.

### 2. Foreign key (FOREIGN KEY)

This is a field that **references the primary key** of another table.
It's what actually creates the relationship between tables.
Example:

```sql
CREATE TABLE orders (
  id INT PRIMARY KEY,
  user_id INT,
  FOREIGN KEY (user_id) REFERENCES users(id)
);
```

Here, `user_id` in `orders` is linked to `id` in `users`.
Every order "belongs to" a specific user.

### 3. Types of relationships between tables

| Relationship type | Description | Example |
|---|---|---|
| One-to-one (1:1) | Each record in table A corresponds to exactly one record in table B. | A user, a passport |
| One-to-many (1:N) | One record in table A corresponds to many records in table B. | A user, orders |
| Many-to-many (M:N) | Each record in A can relate to several in B and vice versa. | Students, courses |

### 4. Why relationships matter

- To **avoid duplicating data**;
- To **enforce integrity** (e.g. you can't create an order without an existing user);
- To **combine data correctly** in `JOIN` queries.

**Summary:**
A relationship between tables is a logical link through keys
that shows **which data relates** to which.
A primary key uniquely identifies a record,
a foreign key points to it from another table,
and the relationships themselves make the database **structured, consistent, and safe**.

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