# Constraints — MySQL Fundamentals: SQL, Joins and Transactions

Source: https://www.skillbyai.com/en/mysql/sql-constraints

> Enforce data quality with UNIQUE, NOT NULL, CHECK and foreign key actions such as ON DELETE CASCADE.

## UNIQUE, CHECK, NOT NULL

Constraints stop bad data before it's ever stored.

```sql
CREATE TABLE products (
  id INT AUTO_INCREMENT PRIMARY KEY,
  sku VARCHAR(20) UNIQUE NOT NULL,
  price DECIMAL(10,2) CHECK (price >= 0)
);
```

## Cascading deletes

`ON DELETE CASCADE` automatically deletes child rows when their parent row is removed.

```sql
CREATE TABLE order_items (
  id INT AUTO_INCREMENT PRIMARY KEY,
  order_id INT NOT NULL,
  product_id INT NOT NULL,
  quantity INT NOT NULL,
  FOREIGN KEY (order_id) REFERENCES orders(id)
    ON DELETE CASCADE
);
```

## Use cascades carefully

Cascading deletes are convenient, but a single `DELETE` on a parent can silently wipe out a lot of related data, so use them with care. `CHECK` constraints are enforced from MySQL 8.0.16.