Urgent SQL Query

OPEN orabaan
declare
@batch int,
@ttype char(3),
@doc int,
@ttdr float,
@ttcr float,
@diff float,
@s1r int,
@s2r int



WHILE
@@FETCH_STATUS = 0

BEGIN
FETCH NEXT FROM orabaan

INTO @batch,
@ttype,
@doc,
@ttdr,
@ttcr,
@diff



if @diff < 0
update
ttfgld102777
set
t_amth_2 = (t_amth_2 + @diff)
where exists
( select top 1 t_amth_2 from orabaan
where
t_dbcr = 1
==> and t_btno = @batch
and t_ttyp = @ttype
and t_docn = @doc )
==============================


I want to add a clause to auto update t_btno when value occur as "t_btno" is variable int


simple data
t_btno t_ttyp t_docn t_dbcr t_amth_2
17 NI1 20050205 1 -3914482498.0
17 NI1 20050205 1 -3914484153.3000002
17 NI1 20050205 2 -3914482497.3000002
17 NI1 20050205 1 -3914482497.3000002
17 NI1 20050205 2 -3914482498.0
17 NI1 20050205 2 -3914484153.3000002

I want to update first row only for each t_btno

thanks regards