TADOQuery and assign null value to the parameter

hi i have the following query:
select yliko.SERIAL_NUMBER,
yliko.PERIGRAFH,yliko.CODE_YPALLHLOY,yliko.CODE_KATASTASHS, YPALLHLOI.NAME,
KATASTASH_YLIKOY.PERIGRAFH, ypallhloi.code
from yliko, ypallhloi, katastash_ylikoy
where (yliko.CODE_YPALLHLOY = ypallhloi.code)
and (yliko.CODE_KATASTASHS = KATASTASH_YLIKOY.code)
and (YPALLHLOI.code= :codeypal or :codeypal is null)
order by ypallhloi.name
the only thing i want to do is to assing values to the parameter using the
folling code:
if ComboYpallhlos.KeyValue<>null then
MhtrvoPagivn.repYlikaYpallhloi.Parameters[0].Value:=FrmYlika_ypallhlo.ComboYpallhlos.KeyValue
else
MhtrvoPagivn.repYlikaYpallhloi.Parameters[0].Value:=null;
my problem is that when my parameter don't have value i get no result... so
i am asking how can i clear my parameter

when i delete the parameter from the sql i get the right results
select yliko.SERIAL_NUMBER,
yliko.PERIGRAFH,yliko.CODE_YPALLHLOY,yliko.CODE_KATASTASHS, YPALLHLOI.NAME,
KATASTASH_YLIKOY.PERIGRAFH, ypallhloi.code
from yliko, ypallhloi, katastash_ylikoy
where (yliko.CODE_YPALLHLOY = ypallhloi.code)
and (yliko.CODE_KATASTASHS = KATASTASH_YLIKOY.code)
order by ypallhloi.name

thank's in advance


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