lesson1Title

lesson2Title

lesson3Title

lesson4Title

lesson5Title

lesson6Title

lesson7Title

lesson8Title

lesson9Title

lesson10Title

lesson11Title

sqlIntroBasicChapter2Title

sqlIntroBasicChapter1Title

sqlIntroBasicChapter3Title

-- View all orders
SELECT * FROM orders;

-- Try to insert a new order with an ID that already exists (will fail)
INSERT INTO orders (order_id, client_id, amount, order_date)
VALUES (101, 3, 150.00, '2023-07-05');

-- View all order items
SELECT * FROM order_items;

-- Try to insert a product that's already in the same order (will fail)
INSERT INTO order_items (order_id, product_id, quantity)
VALUES (101, 501, 1);

# Primary Keys

A **primary key** is a column (or combination of columns) that uniquely identifies each row in a table. It ensures that each row has a *unique key* value and that the key cannot contain `NULL`.

<br/>

## Why Use a Primary Key?

Primary keys are used to:

- Guarantees each record is **unique**
- Prevents duplicate rows
- Enables accurate referencing between tables (e.g. linking clients to orders)

<br/>

## Defining a Primary Key

You can define a primary key when creating a table by using the `PRIMARY KEY` constraint.

```sql title="Create orders table with primary key"
CREATE TABLE orders (
  order_id INT PRIMARY KEY,
  client_id INT,
  amount REAL,
  order_date TEXT
);
```

In this example, `order_id` is the primary key, and each order must have a unique ID.

<br/>

## Composite Primary Key

Sometimes, one column isn't enough to uniquely identify a row. You can combine columns to form a *composite primary key*.

```sql title="Composite primary key"
CREATE TABLE order_items (
  order_id INT,
  product_id INT,
  quantity INT,
  PRIMARY KEY (order_id, product_id)
);
```

This ensures that each product appears only once in an order, preventing duplicates for the same product in the same order.

Primary keys uniquely identify each row and prevent duplicates or NULL values in the key column.

Primary Keys

Why Use a Primary Key?

Defining a Primary Key

Composite Primary Key

Why is a primary key important in SQL tables?