select tablea.fieldx from tablea left join tableb on tablea.fieldx = tableb.fieldy where tableb.fieldx is null
tablea has 8000 records, tableb has 250, fieldx and fieldy are the fields that have the values you want to select/omit.
HTH
ps this would be done through a command object
Edited by lockwelle - 04 May 2011 at 3:48am