--uzywam tempa zeby sobie nie nadpisywac tabeli

IF OBJECT_ID('tempdb..#temp') IS NOT NULL
BEGIN
        DROP TABLE #temp
END

DECLARE @a INT = 1
CREATE TABLE #temp (id int, idlag int, v int)
INSERT INTO #temp (id, idlag, v) (SELECT id, ISNULL(LAG(id) OVER (ORDER BY id), id), v FROM t)

WHILE @a <= (SELECT MAX([id]) FROM #temp)

BEGIN
    SET @a = (SELECT MIN([id]) FROM #temp WHERE v IS NULL)
    UPDATE #temp
    SET v = (SELECT v FROM #temp WHERE id = (SELECT idlag FROM #temp WHERE id = @a))
    WHERE
    id = @a
END

SELECT id, v FROM #temp