I'm not sure what do you want to achieve, but have a look at example:
DECLARE @tmp TABLE (id INT IDENTITY(1,1), Col1 INT, Col2 INT)
INSERT INTO @tmp (Col1)
SELECT 2
UNION ALL SELECT 3
UNION ALL SELECT 1
UNION ALL SELECT 6
UNION ALL SELECT 9
UNION ALL SELECT 8
UNION ALL SELECT 4
UNION ALL SELECT 5
UNION ALL SELECT 7
SELECT id, Col1, Col2 = (SELECT SUM(Col1) AS Col2 FROM @tmp)
FROM @tmp
UPDATE @tmp
SET Col2 = (SELECT SUM(Col1) AS Col2 FROM @tmp)
SELECT id, Col1, Col2
FROM @tmp