MySQL - 값을 모두 더해주는 sum() , 값의 평균을 구해주는 avg()

INSERT INTO books (title, author_fname, author_lname, released_year, stock_quantity, pages)
VALUES
('The Namesake', 'Jhumpa', 'Lahiri', 2003, 32, 291),
('Norse Mythology', 'Neil', 'Gaiman',2016, 43, 304),
('American Gods', 'Neil', 'Gaiman', 2001, 12, 465),
('Interpreter of Maladies', 'Jhumpa', 'Lahiri', 1996, 97, 198),
('A Hologram for the King: A Novel', 'Dave', 'Eggers', 2012, 154, 352),
('The Circle', 'Dave', 'Eggers', 2013, 26, 504),
('The Amazing Adventures of Kavalier & Clay', 'Michael', 'Chabon', 2000, 68, 634),
('Just Kids', 'Patti', 'Smith', 2010, 55, 304),
('A Heartbreaking Work of Staggering Genius', 'Dave', 'Eggers', 2001, 104, 437),
('Coraline', 'Neil', 'Gaiman', 2003, 100, 208),
('What We Talk About When We Talk About Love: Stories', 'Raymond', 'Carver', 1981, 23, 176),
("Where I'm Calling From: Selected Stories", 'Raymond', 'Carver', 1989, 12, 526),
('White Noise', 'Don', 'DeLillo', 1985, 49, 320),
('Cannery Row', 'John', 'Steinbeck', 1945, 95, 181),
('Oblivion: Stories', 'David', 'Foster Wallace', 2004, 172, 329),
('Consider the Lobster', 'David', 'Foster Wallace', 2005, 92, 343),
('10% Happier', 'Dan', 'Harris', 2014, 29, 256),
('fake_book', 'Freida', 'Harris', 2001, 287, 428),
('Lincoln In the Bardo', 'George', 'Saunders', 2017, 156, 375);

값을 모두 더해주는 sum() 함수

-- books 테이블의 모든 책 페이지수를 다 더하면??

select sum(pages)
from books;

-- 각 작가별로 자신이 쓴 책의 페이지 수를 모두 더해서 보여주시오.

select concat(author_fname,' ',author_lname),sum(pages)
from books
group by author_lname,author_fname;

평균 구하는 함수 avg()

-- books 테이블의 페이지 수의 평균을 구하시오.

select avg(pages)
from books;

-- 년도 별로 재고의 평균을 구하시오. 년도와 재고평균만 나오도록 하시오.

select released_year,avg(stock_quantity)
from books
group by released_year;