You can use a subquery. The subquery will get the Max(CompletedDate)
. You then take this value and join on your table again to retrieve the note associate with that date:
select ET1.TrainingID,
ET1.CompletedDate,
ET1.Notes
from HR_EmployeeTrainings ET1
inner join
(
select Max(CompletedDate) CompletedDate, TrainingID
from HR_EmployeeTrainings
--where AvantiRecID IS NULL OR AvantiRecID = @avantiRecID
group by TrainingID
) ET2
on ET1.TrainingID = ET2.TrainingID
and ET1.CompletedDate = ET2.CompletedDate
where ET1.AvantiRecID IS NULL OR ET1.AvantiRecID = @avantiRecID