why this query does not work

[admin]

i got a query like this:

UPDATE submissions
set sub_status = 'Normal'
where sub_id = (select submissions.sub_id
from submissions, assignments where submissions.assg_no = assignments.assg_no
and UNIX_TIMESTAMP(submissions.sub_datetime) <= UNIX_TIMESTAMP(assignments.soft_datetime));

i try to run this but the system always give me a syntax error.

please help

kelvin