SQL problemático

function RetSQLSituacaoOperacao: string;
begin
  Result := 'select top 1 usu_sitorp ' + sLineBreak +
    'from usu_tcabpro ' + sLineBreak +
    'where usu_codemp = :usu_codemp ' + sLineBreak +
    '  and usu_codori = :usu_codori ' + sLineBreak +
    '  and usu_codopr = :usu_codopr ' + sLineBreak +
//    'order by usu_datfim desc, usu_horfim desc, usu_datini desc, usu_horini desc ';
    'order by (case when usu_datfim is null then 0 else 1 end), usu_datfim desc, (case when usu_horfim is null then 0 else 1 end), usu_horfim desc, usu_datini desc, usu_horini desc ';
end;