id (int)
udid (int)
assid (int)
id (int)
assid (int)
update u
set u.assid = s.assid
from ud u
inner join sale s on
u.id = s.udid
UPDATE ud
SET assid = (SELECT assid FROM sale s WHERE ud.id=s.id)
On SQL Server you can use a join
UPDATE ud
SET assid = s.assid
FROM ud u
JOIN sale s ON u.id=s.id