Complex(?) Query

Hello experts.

I'm a novice sql writer and need some help in writing a query to
extract applicable data from the following table (titled EMPLOYEE):

--
ID_NUMBERCODEDATE
------------------ --------- --------
12 VO20060914
12 XD20060913
12 AD20060912
12 WR20060911
12 AT20060910
45 VO20060914
45 XR20060913
45 AT20060912
45 AD20060911
45 AT20060910
78 AD20060914
78 AT20060913
78 VO20060912
78 AD20060911
78 AT20060910

I need to select ID_NUMBER
from EMPLOYEE
where CODE = 'VO'

caveat: I only want the ID_NUMBER(s) where the CODE = 'VO'
and the previous CODE (by DATE) = 'AD'
or the previous CODE (by DATE) = 'AD' with any CODE in between
except 'AT';

E.g., in the above example, the appropriate code should select
ID_NUMBER(s) 12 and 78 because
1. a VO code exists
2. an AD code (by DATE) precedes it
3. although 'AD' does not come immediately before 'VO' (in the
case of ID_NUMBER 12) 'AT' cannot be found in between

I hope I haven't confused anyone. Any help would be appreciated.