You can use the
COALESCE
expression
MSDN Coalesce[
^]
eg
SELECT l._application_id_c, p.ID, COALESCE(l.datetime_c, p.ApplicationDate) as datetime_c
FROM dbo.leads l
LEFT OUTER JOIN dbo.Applications p ON l.application_id_c = p.ApplicationID
[Edit - prompted by a comment from Prasad Khandekar I've given the full sql statement