List the names of all the distinct customers who bought a keyboard.
select distinct t1.customer_name from customers as t1 join customer_orders as t2 on t1.customer_id = t2.customer_id join order_items as t3 on t2.order_id = t3.order_id join products as t4 on t3.product_id = t4.product_id where t4.product_name = "keyboard"