先把两张表连接，amount为null 的正是我们需要的，再按customer_id聚合

select  Visits.visit_id,customer_id ,Transactions.visit_id ,transaction_id ,amount 
from Visits left join Transactions on Visits.visit_id=Transactions.visit_id

正确代码： 

select customer_id , count(customer_id ) count_no_trans 
from Visits left join Transactions 
on Visits.visit_id = Transactions.visit_id 
where amount is null
group by customer_id