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

SELECT id_customer, COUNT(DISTINCT id_customer) * 100 / (SELECT COUNT(DISTINCT id_customer) FROM sales) FROM sales WHERE year(created_at) = 2020 AND id_customer NOT IN (SELECT id_customer FROM sales WHERE year(created_at) = 2019) GROUP BY id_customer