get percentages on SQL querry - Oracle

If we want to get the percentage of some column on same query, we can do it using "ratio_to_report() over()" function.

ratio_to_report(Number_of_books) over()

let's say we have following table called tbl_books.

Book	Number_of_books
Book1	30
Book1	20
Book1	10

if we want to get percentages of each book over total number of books we can do as following,

select book,ratio_to_report(Number_of_books) over() * 100 as percentage from tbl_books

it gives,

Book	percentage
Book1	50
Book1	33.333333333
Book1	16.66666666