There is a simple example here, including sample:
http://support.microsoft.com/default.aspx?scid=kb;en-us;209066&Product=acc2000
Also:
http://support.microsoft.com/default.aspx?scid=kb;en-us;208953&Product=acc2000
http://support.microsoft.com/default.aspx?scid=kb;en-us;210039&Product=acc2000
They work well in Access, and are very useful. Main limitations are that the
query is read-only when you use a subquery in the field list, and you can
run into "multi-level group-by" errors on reports that use subqueries in
their RecordSource.
As an example of non-trivial use of subqueries, a not-for-profit
organization wanted to be able to select donors for a mailing based on just
about any combination of criteria in any other related tables (donors,
donations, pledges, mailings, mail-types, segmentation, demographics,
notations, roles, etc, etc.). We created an unbound form with 9 tabs full of
text boxes, list boxes, etc, where the user could enter criteria against
most of the fields in most of the tables, and then dynamically built a SQL
statement full of subqueries to select the appropriate donors for the
mailing.