Maybe not that you need, but this works

DECLARE

@id int,

@weight int,

@Field varchar(12),

@Result int

-- Test values

--SET @iD = 3

--SET @weight = 28

IF NOT EXISTS(SELECT id FROM [dbo].[weights] WHERE iD = @id)

BEGIN

INSERT INTO [dbo].[weights](id,shipweight1)

VALUES(@id,@weight)

END

ELSE BEGIN

SELECT @Result = shipweight2 FROM [dbo].[weights] WHERE iD = @iD

SELECT @Field =

CASE

WHEN @Result <> 0 THEN 'shipweight3'

ELSE 'shipweight2'

END

IF @Field = 'shipweight2' BEGIN

UPDATE [dbo].[weights]

SET shipweight2 = @weight

WHERE iD = @iD

END

ELSE BEGIN

-- check first if is null, for not overwrite

SELECT @Result = shipweight3 FROM [dbo].[weights] WHERE iD = @iD

IF @Result is null BEGIN

UPDATE [dbo].[weights]

SETshipweight3 = @weight

WHERE iD = @iD

END

END

END

-- SELECT * FROM [dbo].[weights] WHERE iD = @iD