CREATE TABLE IF NOT EXISTS user_assets (
    id INT AUTO_INCREMENT PRIMARY KEY,
    user_id INT NOT NULL,
    symbol VARCHAR(20) NOT NULL,
    amount DECIMAL(30, 10) DEFAULT 0,
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    updated_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
    UNIQUE KEY user_symbol (user_id, symbol)
);