$select = "SELECT name_f, id_f, val_v,
case val_v
when '0' then price_v/$dkr
when '2' then price_v*$ekd
else price_v end as newf";
$from = " FROM firms, country ...";
$where = " WHERE visa.id_see_v = '1' and ... ";

if ($idc != "") { $where .= " AND id_country_v = $idc";}
if ($nf != "") { $where .= " AND name_f LIKE '%$nf%'";}

if ($f != "") {$where .= "order by name_f"; }
else {$where .= "HAVING newf <= '$cena'"; }

все работает супер, но если не передается необязательная переменная типа$idc

if ($idc != "") { $where .= " AND id_country_v = $idc";}
, а вот $nf работает нормально

if ($nf != "") { $where .= " AND name_f LIKE '%$nf%'";}