Explain PostgreSQL constraint types and their usage.

Constraint Types:

PRIMARY KEY:
• Unique identifier
• Cannot be NULL
• Automatically creates index

FOREIGN KEY:
• References another table
• Maintains referential integrity
• Supports CASCADE actions

UNIQUE:
• Ensures uniqueness
• Allows one NULL value

CHECK:
• Custom validation rules
• Boolean expressions

NOT NULL:
• Prevents NULL values

Example:

CREATE TABLE orders (
    id SERIAL PRIMARY KEY,
    user_id INT REFERENCES users(id) ON DELETE CASCADE,
    amount DECIMAL CHECK (amount > 0),
    email VARCHAR UNIQUE NOT NULL
);