Lynn is absolutely correct that in order to give any solid advice we need a lot more details to work with. However, maybe I can give you a nudge that will help you. It seems that what you need to do is to put together a cte that has all the CustomerID values that were in attendance. Then you can query your customer table with a left join to your cte. If the cte value is NULL they did not attend, otherwise they did.

Something like this maybe:

with Attended as

(

select jh.SomeID

from jh

join Web on [some conditions]

)

select case when a.SomeID IS NOT NULL then 'Attended' else 'Did Not Attend' end

from jh

left join Attended a on a.SomeID = jh.SomeID