SQL Server: IF EXISTS ; ELSE

I have a tableA:

ID value
 1  100
 2  101
 2  444
 3  501

Also TableB

ID Code
1
2

Now I want to populate col = code of table B if there exists ID = 2 in tableA. for multiple values , get max value. else populate it with '123'. Now here is what I used:

if exists (select MAX(value) from #A where id = 2)
 BEGIN
 update #B
 set code = (select MAX(value) from #A where id = 2)
 from #A
 END

 ELSE 

 update #B
 set code = 123
 from #B

I am sure there is some problem in BEGIN;END or in IF EXIST;ELSE. Basically I want to by-pass the else part if select statement in IF-part exist and vice- versa. For example if select statement of IF=part is:

(select MAX(value) from #A where id = 4)

It should just populate 123, coz ID = 4 do not exist !

EDIT

I want to add the reason that your IF statement seems to not work. When you do an EXISTS on an aggregate, it's always going to be true. It returns a value even if the ID doesn't exist. Sure, it's NULL, but its returning it. Instead, do this:

if exists(select 1 from table where id = 4)

and you'll get to the ELSE portion of your IF statement.

Now, here's a better, set-based solution:

update b
  set code = isnull(a.value, 123)
from #b b
left join (select id, max(value) from #a group by id) a
  on b.id = a.id
where
  b.id = yourid

This has the benefit of being able to run on the entire table rather than individual ids.

Try this:

Update TableB Set
  Code = Coalesce(
    (Select Max(Value)
    From TableA 
    Where Id = b.Id), 123)
From TableB b