### Database Schema
CREATE TABLE actor (
  actor_id SMALLINT UNSIGNED NOT NULL,
  first_name VARCHAR(45) NOT NULL,
  last_name VARCHAR(45) NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (actor_id)
)

CREATE TABLE address (
  address_id SMALLINT UNSIGNED NOT NULL,
  address VARCHAR(50) NOT NULL,
  address2 VARCHAR(50) DEFAULT NULL,
  district VARCHAR(20) NOT NULL,
  city_id SMALLINT UNSIGNED NOT NULL,
  postal_code VARCHAR(10) DEFAULT NULL,
  phone VARCHAR(20) NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (address_id),
  FOREIGN KEY (city_id) REFERENCES city (city_id)
)

CREATE TABLE category (
  category_id TINYINT UNSIGNED NOT NULL,
  name VARCHAR(25) NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (category_id)
)

CREATE TABLE city (
  city_id SMALLINT UNSIGNED NOT NULL,
  city VARCHAR(50) NOT NULL,
  country_id SMALLINT UNSIGNED NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (city_id),
  FOREIGN KEY (country_id) REFERENCES country (country_id)
)

CREATE TABLE country (
  country_id SMALLINT UNSIGNED NOT NULL,
  country VARCHAR(50) NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (country_id)
)

CREATE TABLE customer (
  customer_id SMALLINT UNSIGNED NOT NULL,
  store_id TINYINT UNSIGNED NOT NULL,
  first_name VARCHAR(45) NOT NULL,
  last_name VARCHAR(45) NOT NULL,
  email VARCHAR(50) DEFAULT NULL,
  address_id SMALLINT UNSIGNED NOT NULL,
  active BOOLEAN NOT NULL DEFAULT TRUE,
  create_date DATETIME NOT NULL,
  last_update TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (customer_id),
  FOREIGN KEY (address_id) REFERENCES address (address_id),
  FOREIGN KEY (store_id) REFERENCES store (store_id)
)

CREATE TABLE film (
  film_id SMALLINT UNSIGNED NOT NULL,
  title VARCHAR(255) NOT NULL,
  description TEXT DEFAULT NULL,
  release_year YEAR DEFAULT NULL,
  language_id TINYINT UNSIGNED NOT NULL,
  original_language_id TINYINT UNSIGNED DEFAULT NULL,
  rental_duration TINYINT UNSIGNED NOT NULL DEFAULT 3,
  rental_rate DECIMAL(4,2) NOT NULL DEFAULT 4.99,
  length SMALLINT UNSIGNED DEFAULT NULL,
  replacement_cost DECIMAL(5,2) NOT NULL DEFAULT 19.99,
  rating DEFAULT 'G',
  special_features DEFAULT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (film_id),
  FOREIGN KEY (language_id) REFERENCES language (language_id),
  FOREIGN KEY (original_language_id) REFERENCES language (language_id)
)

CREATE TABLE film_actor (
  actor_id SMALLINT UNSIGNED NOT NULL,
  film_id SMALLINT UNSIGNED NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (actor_id,film_id),
 FOREIGN KEY (actor_id) REFERENCES actor (actor_id),
  FOREIGN KEY (film_id) REFERENCES film (film_id)
)

CREATE TABLE film_category (
  film_id SMALLINT UNSIGNED NOT NULL,
  category_id TINYINT UNSIGNED NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (film_id, category_id),
  FOREIGN KEY (film_id) REFERENCES film (film_id),
  FOREIGN KEY (category_id) REFERENCES category (category_id)
)

CREATE TABLE film_text (
  film_id SMALLINT NOT NULL,
  title VARCHAR(255) NOT NULL,
  description TEXT,
  PRIMARY KEY  (film_id)
)

CREATE TABLE inventory (
  inventory_id MEDIUMINT UNSIGNED NOT NULL,
  film_id SMALLINT UNSIGNED NOT NULL,
  store_id TINYINT UNSIGNED NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (inventory_id),
  FOREIGN KEY (store_id) REFERENCES store (store_id),
  FOREIGN KEY (film_id) REFERENCES film (film_id)
)

CREATE TABLE language (
  language_id TINYINT UNSIGNED NOT NULL,
  name CHAR(20) NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (language_id)
)

CREATE TABLE payment (
  payment_id SMALLINT UNSIGNED NOT NULL,
  customer_id SMALLINT UNSIGNED NOT NULL,
  staff_id TINYINT UNSIGNED NOT NULL,
  rental_id INT DEFAULT NULL,
  amount DECIMAL(5,2) NOT NULL,
  payment_date DATETIME NOT NULL,
  last_update TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (payment_id),
  FOREIGN KEY (rental_id) REFERENCES rental (rental_id),
  FOREIGN KEY (customer_id) REFERENCES customer (customer_id),
  FOREIGN KEY (staff_id) REFERENCES staff (staff_id)
)

CREATE TABLE rental (
  rental_id INT NOT NULL,
  rental_date DATETIME NOT NULL,
  inventory_id MEDIUMINT UNSIGNED NOT NULL,
  customer_id SMALLINT UNSIGNED NOT NULL,
  return_date DATETIME DEFAULT NULL,
  staff_id TINYINT UNSIGNED NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (rental_id),
  FOREIGN KEY (staff_id) REFERENCES staff (staff_id),
  FOREIGN KEY (inventory_id) REFERENCES inventory (inventory_id),
  FOREIGN KEY (customer_id) REFERENCES customer (customer_id)
)

CREATE TABLE staff (
  staff_id TINYINT UNSIGNED NOT NULL,
  first_name VARCHAR(45) NOT NULL,
  last_name VARCHAR(45) NOT NULL,
  address_id SMALLINT UNSIGNED NOT NULL,
  picture BLOB DEFAULT NULL,
  email VARCHAR(50) DEFAULT NULL,
  store_id TINYINT UNSIGNED NOT NULL,
  active BOOLEAN NOT NULL DEFAULT TRUE,
  username VARCHAR(16) NOT NULL,
  password VARCHAR(40) DEFAULT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (staff_id),
  --FOREIGN KEY (store_id) REFERENCES store (store_id),
  FOREIGN KEY (address_id) REFERENCES address (address_id)
)

CREATE TABLE store (
  store_id TINYINT UNSIGNED NOT NULL,
  manager_staff_id TINYINT UNSIGNED NOT NULL,
  address_id SMALLINT UNSIGNED NOT NULL,
  last_update TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY  (store_id),
  FOREIGN KEY (manager_staff_id) REFERENCES staff (staff_id),
  FOREIGN KEY (address_id) REFERENCES address (address_id)
)

### Question
Return the sum of all payment amounts.

### SQL
SELECT sum(amount) FROM payment