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

SELECT count(distinct(customers.id))*100/(select count(*) FROM customers) FROM customers inner join purchases on customers.id=purchases.customer_id inner join branches on purchases.branch_id=branches.id inner join customers on customers.id=purchases.customer_id where YEAR(purchases.created_at)=2020;