select count(distinct attendance.studentid)
from attendance
where attendance.studentid not in (select studentid from attendance where auth not in('C','E','F','G','H','I','M','N','O','R','S','T','U'))
I am tired a lot now, but isn`t it the same as:
select count(distinct attendance.studentid)
from attendance
where auth in('C','E','F','G','H','I','M','N','O','R','S','T','U')
?