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-- View all clients
SELECT * FROM clients;

-- Insert a client without a status (DEFAULT will apply: 'active')
INSERT INTO clients (id, email, name)
VALUES (6, 'newclient@example.com', 'New Client');

-- View all clients
SELECT * FROM clients;

-- Attempt to insert a client with a duplicate email (will fail due to UNIQUE constraint)
INSERT INTO clients (id, email, name)
VALUES (7, 'alex@example.com', 'Duplicate Entry');

# Unique and Default Constraints

SQL allows you to define **constraints** on your columns to enforce rules. Two common constraints are:

- `UNIQUE`: ensures all values in a column are different  
- `DEFAULT`: provides an automatic value if none is specified

<br/>

## UNIQUE Constraint

The `UNIQUE` constraint ensures no two rows in a column have the same value.

```sql title="UNIQUE constraint on email"
CREATE TABLE clients (
  id INT PRIMARY KEY,
  email TEXT UNIQUE,
  name TEXT,
  status TEXT DEFAULT 'active'
);
```

In this table, the `email` column is `UNIQUE`, meaning only one client can use `alex@example.com`.

<br/>

## DEFAULT Constraint

The `DEFAULT` constraint sets a value automatically when none is given.

```sql title="Create table with DEFAULT constraint"
CREATE TABLE clients (
  id INT PRIMARY KEY,
  email TEXT UNIQUE,
  name TEXT,
  status TEXT DEFAULT 'active'
);
```

Here, `status` will default to `'active'` unless you provide something else.

<br/>

## INSERT INTO Example

The query below inserts a new client named Laura Adams.

```sql title="Insert using default value"
INSERT INTO clients (id, email, name)
VALUES (6, 'laura.adams@newdomain.com', 'Laura Adams');
```

If you don't specify a `status`, the default value `'active'` will be used.

<br/>

## Why It Matters

Constraints help prevent duplicate records (such as duplicate emails), automatically fill in missing values, and keep your data clean and reliable.

The UNIQUE constraint ensures that all values in a specific column are distinct.

The DEFAULT constraint provides a fallback value if no explicit value is provided for a column.

Unique and Default Constraints

UNIQUE Constraint

DEFAULT Constraint

INSERT INTO Example

Why It Matters

What does the `UNIQUE` constraint do in SQL?

What does the `DEFAULT` constraint do in SQL?