[sql] 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 !

Try this:

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

I know its been a while since the original post but I like using CTE's and this worked for me:

WITH cte_table_a
AS
(
    SELECT [id] [id]
    , MAX([value]) [value]
    FROM table_a
    GROUP BY [id]
)
UPDATE table_b
SET table_b.code = CASE WHEN cte_table_a.[value] IS NOT NULL THEN cte_table_a.[value] ELSE 124 END
FROM table_b
LEFT OUTER JOIN  cte_table_a
ON table_b.id = cte_table_a.id