How to set variable in to Ms Sql select

Question

How can I set in MS Sql select variables, idea looks like this:

Declare @var int

SET @var = 0;

Select (if(idid = @var) then sum+1 else sum-1) AS Sum,
   set @var = id 
FROM table

Edit based on comments.

DECLARE @T TABLE
(
ID  INT PRIMARY KEY,
IDID INT,
SUMM INT 
)

INSERT INTO @T
SELECT 1,1,4 UNION ALL
SELECT 2,1,5 UNION ALL 
SELECT 3,2,6 UNION ALL 
SELECT 4,2,7 UNION ALL 
SELECT 5,3,8

In select result I need:

ID  IDID  SUMM 
--  ----  ----
1   1     4 
2   1     0 
3   2     6 
4   2     0 
5   3     8

Answer 1

Is this what you are trying to do?

Declare @var int, @id int
SET @var = 0

SELECT @id = id FROM Table t

if(@id = @var)
BEGIN
    SET @var = @var + 1
END
ELSE
BEGIN
    SET @var = @var - 1
END

print @var