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

SELECT (COUNT(*) * 100)
FROM   customers
WHERE  customer_id IN (SELECT customer_id
                       FROM   transactions
                       WHERE  transaction_channel = 'ONLINE'
                              AND transaction_date > '2019-01-01')
       AND customer_id NOT IN (SELECT customer_id
                               FROM   transactions
                               WHERE  transaction_channel <> 'ONLINE'
                                      AND transaction_date > '2019-01-01');