You are now a sqlite data analyst, and you are given a database schema as follows:

    【Schema】
    【DB_ID】 movie_3
【Schema】
# Table: country
[
(country_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(country:TEXT, Examples: [Afghanistan, Algeria, American Samoa]),
(last_update:DATETIME, Examples: [2006-02-15 04:44:00])
]
# Table: customer
[
(customer_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(store_id:INTEGER, Examples: [1, 2]),
(first_name:TEXT, Examples: [MARY, PATRICIA, LINDA]),
(last_name:TEXT, Examples: [SMITH, JOHNSON, WILLIAMS]),
(email:TEXT),
(address_id:INTEGER, Examples: [5, 6, 7]),
(active:INTEGER, Examples: [1, 0]),
(create_date:DATETIME, Examples: [2006-02-14 22:04:36]),
(last_update:DATETIME, Examples: [2006-02-15 04:57:20])
]
# Table: film_text
[
(film_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(title:TEXT, Examples: [ACADEMY DINOSAUR, ACE GOLDFINGER, ADAPTATION HOLES]),
(description:TEXT)
]
# Table: store
[
(store_id:INTEGER, Primary Key, Examples: [1, 2]),
(manager_staff_id:INTEGER, Examples: [1, 2]),
(address_id:INTEGER, Examples: [1, 2]),
(last_update:DATETIME, Examples: [2006-02-15 04:57:12])
]
# Table: film_category
[
(film_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(category_id:INTEGER, Primary Key, Examples: [6, 11, 8]),
(last_update:DATETIME, Examples: [2006-02-15 05:07:09])
]
# Table: payment
[
(payment_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(customer_id:INTEGER, Examples: [1, 2, 3]),
(staff_id:INTEGER, Examples: [1, 2]),
(rental_id:INTEGER, Examples: [76, 573, 1185]),
(amount:REAL, Examples: [2.99, 0.99, 5.99]),
(payment_date:DATETIME, Examples: [2005-05-25 11:30:37]),
(last_update:DATETIME, Examples: [2006-02-15 22:12:30])
]
# Table: actor
[
(actor_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(first_name:TEXT, Examples: [PENELOPE, NICK, ED]),
(last_name:TEXT, Examples: [GUINESS, WAHLBERG, CHASE]),
(last_update:DATETIME, Examples: [2006-02-15 04:34:33])
]
# Table: rental
[
(rental_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(rental_date:DATETIME, Examples: [2005-05-24 22:53:30]),
(inventory_id:INTEGER, Examples: [367, 1525, 1711]),
(customer_id:INTEGER, Examples: [130, 459, 408]),
(return_date:DATETIME, Examples: [2005-05-26 22:04:30]),
(staff_id:INTEGER, Examples: [1, 2]),
(last_update:DATETIME, Examples: [2006-02-15 21:30:53])
]
# Table: language
[
(language_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(name:TEXT, Examples: [English, Italian, Japanese]),
(last_update:DATETIME, Examples: [2006-02-15 05:02:19])
]
# Table: film
[
(film_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(title:TEXT, Examples: [ACADEMY DINOSAUR, ACE GOLDFINGER, ADAPTATION HOLES]),
(description:TEXT),
(release_year:TEXT, Examples: [2006]),
(language_id:INTEGER, Examples: [1]),
(original_language_id:INTEGER),
(rental_duration:INTEGER, Examples: [6, 3, 7]),
(rental_rate:REAL, Examples: [0.99, 4.99, 2.99]),
(length:INTEGER, Examples: [86, 48, 50]),
(replacement_cost:REAL, Examples: [20.99, 12.99, 18.99]),
(rating:TEXT, Examples: [PG, G, NC-17]),
(special_features:TEXT, Examples: [Deleted Scenes,Behind the Scenes]),
(last_update:DATETIME, Examples: [2006-02-15 05:03:42])
]
# Table: inventory
[
(inventory_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(film_id:INTEGER, Examples: [1, 2, 3]),
(store_id:INTEGER, Examples: [1, 2]),
(last_update:DATETIME, Examples: [2006-02-15 05:09:17])
]
# Table: address
[
(address_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(address:TEXT, Examples: [47 MySakila Drive, 28 MySQL Boulevard, 23 Workhaven Lane]),
(address2:TEXT),
(district:TEXT, Examples: [Alberta, QLD, Nagasaki]),
(city_id:INTEGER, Examples: [300, 576, 463]),
(postal_code:TEXT, Examples: [35200, 17886, 83579]),
(phone:TEXT, Examples: [14033335568, 6172235589, 28303384290]),
(last_update:DATETIME, Examples: [2006-02-15 04:45:30])
]
# Table: film_actor
[
(actor_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(film_id:INTEGER, Primary Key, Examples: [1, 23, 25]),
(last_update:DATETIME, Examples: [2006-02-15 05:05:03])
]
# Table: category
[
(category_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(name:TEXT, Examples: [Action, Animation, Children]),
(last_update:DATETIME, Examples: [2006-02-15 04:46:27])
]
# Table: city
[
(city_id:INTEGER, Primary Key, Examples: [1, 2, 3]),
(city:TEXT, Examples: [A Corua (La Corua), Abha, Abu Dhabi]),
(country_id:INTEGER, Examples: [87, 82, 101]),
(last_update:DATETIME, Examples: [2006-02-15 04:45:25])
]
# Table: staff
[
(staff_id:INTEGER, Primary Key, Examples: [1, 2]),
(first_name:TEXT, Examples: [Mike, Jon]),
(last_name:TEXT, Examples: [Hillyer, Stephens]),
(address_id:INTEGER, Examples: [3, 4]),
(picture:BLOB),
(email:TEXT),
(store_id:INTEGER, Examples: [1, 2]),
(active:INTEGER, Examples: [1]),
(username:TEXT, Examples: [Mike, Jon]),
(password:TEXT, Examples: [8cb2237d0679ca88db6464eac60da96345513964]),
(last_update:DATETIME, Examples: [2006-02-15 04:57:16])
]
【Foreign keys】
customer.address_id=address.address_id
customer.store_id=store.store_id
store.address_id=address.address_id
store.manager_staff_id=staff.staff_id
film_category.category_id=category.category_id
film_category.film_id=film.film_id
payment.rental_id=rental.rental_id
payment.staff_id=staff.staff_id
payment.customer_id=customer.customer_id
rental.staff_id=staff.staff_id
rental.customer_id=customer.customer_id
rental.inventory_id=inventory.inventory_id
film.original_language_id=language.language_id
film.language_id=language.language_id
inventory.store_id=store.store_id
inventory.film_id=film.film_id
address.city_id=city.city_id
film_actor.film_id=film.film_id
film_actor.actor_id=actor.actor_id
city.country_id=country.country_id
staff.store_id=store.store_id
staff.address_id=address.address_id

    【Question】
    Calculate the average payment amount per customer. average payment refers to AVG(amount)

    【Evidence】
    average payment refers to AVG(amount)

    Please read and understand the database schema carefully, and generate an executable SQL based on the user's question and evidence. 

SQL: SELECT AVG(amount) FROM payment GROUP BY customer_id