Del total de clientes. Qué porcentaje realizó compras sólo por el canal online entre 2019 y 2020

SELECT (COUNT(*) / (SELECT COUNT(*) FROM customers WHERE (channel=1) AND (purchase_date BETWEEN '2019-01-01' AND '2020-12-31'))) * 100 FROM customers WHERE (channel=1) AND (purchase_date BETWEEN '2019-01-01' AND '2020-12-31');