Del total de clientes que realizaron compras en 2020. Qué porcentaje no había realizado compras en 2019. Tenido la tabla de cliente, compra

select count(*) as new_clients from customers where id_customer in (select id_customer from purchases where year(date_purchase) = 2020 and id_customer not in (select id_customer from purchases where year(date_purchase) = 2019));