DECLARE @i int, @l int;
SELECT
    @i = 12, @l = LEN(@i);
    
SELECT 
    SUM(x.Digit)
FROM
    (SELECT
        CAST(SUBSTRING(RTRIM(@i), N.Number, 1) AS int) AS Digit
    FROM
        dbo.Number N
    WHERE 
        N.Number BETWEEN 1 AND LEN(RTRIM(@i))
    ) x;