Del total de clientes que realizaron compras en 2020. Que porcentaje lo hizo en una sola sucursal. Teniendo tabla, compra, sucursal, cliente

SELECT t1.total, t2.cantidad FROM
(
    SELECT COUNT(*) total FROM (SELECT id_cliente  FROM sucursal WHERE id_cliente IS NOT NULL GROUP BY id_cliente, id_sucursal HAVING COUNT(id_cliente) > 1) tabla1
) t1
CROSS JOIN 
(
    SELECT COUNT(*) cantidad FROM (SELECT id_cliente  FROM sucursal WHERE id_cliente IS NOT NULL GROUP BY id_cliente) tabla2
) t2