SELECT AVG(released_year) 
FROM books;

SELECT AVG(pages) 
FROM books;

SELECT AVG(stock_quantity) 
FROM books 
GROUP BY released_year;

SELECT released_year, AVG(stock_quantity) 
FROM books 
GROUP BY released_year;

SELECT author_fname, author_lname, AVG(pages) FROM books
GROUP BY author_lname, author_fname;