Triggers with OLD and NEW Values

I need to create a trigger which will update the same table column with the other coulmn of the tables if the other column had a NULL old vlaue .something like below
Create trigger count_Dept
ON Dept
after insert or update
for each row
begin
IF (:old.count1 is null and :new.count2 is not null)
update dept set count2= :new.count1
where  :old.count1 is null and :new.count2 is not null
end ;
We can do something like this in oracle how can we do it in SQL server ????