query takes a long time

[Garry Saddington <garry@xxxxxxxxxxxxxxxxxxxx>]

Can someone suggest how I could speed up the execution of this query?

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'))

It returns the correct results but very slowly.

regards

garry