안녕하세요.

만들어진 검색폼을 다른곳에 응용하려 하는데 잘안되는 부분이 있어서 질문드립니다.
아래와 같이 선택항목으로 검색을 하는데요 '직업'을 선택하면 검색이 제대로 이루어 집니다.
그런데 '나이' 부분만을 선택하면 검색이 되질 안습니다. 그냥 결과에 없는 것으로 나오네요.
그리고 직업과 나이 두개 조건을 선택하면 선택에 맞게 검색이 이루어집니다.
직업항목을 선택하지 안더라도 검색이 이루어 지게 하려면 어찌해야 되는지요.?

<td bgcolor="#FFFFFF" width="66" height="24">
                            <select name="mb_age">
                                <option value="" <? if($mb_age == "") { echo "selected"; } ?> >-나 이-</option>
                                <option value="10" <? if($mb_age == "10") { echo "selected"; } ?> >10~19</option>
                                <option value="20" <? if($mb_age == "20") { echo "selected"; } ?> >20~29</option>
                                <option value="30" <? if($mb_age == "30") { echo "selected"; } ?> >30~39</option>
                                <option value="40" <? if($mb_age == "40") { echo "selected"; } ?> >40~49</option>
                                <option value="50" <? if($mb_age == "50") { echo "selected"; } ?> >50~59</option>
                                <option value="60" <? if($mb_age == "60") { echo "selected"; } ?> >60~69</option>
                                <option value="70" <? if($mb_age == "70") { echo "selected"; } ?> >70~79</option>
                                <option value="80" <? if($mb_age == "80") { echo "selected"; } ?> >80~89</option>
                              </select>
                          </td>
                          <td bgcolor="#FFFFFF" width="6" height="24">&nbsp;</td>
                          <td bgcolor="#FFFFFF" width="130" height="24">
                            <select name="mb_job[]">
                              <option value="" <? if($mb_job == "") { echo "selected"; } ?> > -직 업-</option>
                              <option value="A1" <? if($mb_job == "A1") { echo "selected"; } ?> >중소기업CEO</option>
                              <option value="A2" <? if($mb_job == "A2") { echo "selected"; } ?> >대기업임원</option>
                              <option value="A3" <? if($mb_job == "A3") { echo "selected"; } ?> >대기업직원</option>
                              <option value="A4" <? if($mb_job == "A4") { echo "selected"; } ?> >회사원</option>
                              <option value="A5" <? if($mb_job == "A5") { echo "selected"; } ?> >변호사</option>
                              <option value="A6" <? if($mb_job == "A6") { echo "selected"; } ?> >변리사</option>
                              <option value="A7" <? if($mb_job == "A7") { echo "selected"; } ?> >세무사</option>
                              <option value="A8" <? if($mb_job == "A8") { echo "selected"; } ?> >회계사</option>
                              <option value="A9" <? if($mb_job == "A9") { echo "selected"; } ?> >의사</option>
                              <option value="B1" <? if($mb_job == "B1") { echo "selected"; } ?> >수의사</option>
                              <option value="B2" <? if($mb_job == "B2") { echo "selected"; } ?> >치과의사</option>
                              <option value="B3" <? if($mb_job == "B3") { echo "selected"; } ?> >자영업</option>
                              <option value="B4" <? if($mb_job == "B4") { echo "selected"; } ?> >교수</option>
                              <option value="B5" <? if($mb_job == "B5") { echo "selected"; } ?> >공무원</option>
                              <option value="B6" <? if($mb_job == "B6") { echo "selected"; } ?> >정부투자기관임원</option>
                              <option value="B7" <? if($mb_job == "B7") { echo "selected"; } ?> >정부투자기관직원</option>
                              <option value="B8" <? if($mb_job == "B8") { echo "selected"; } ?> >예술인 </option>
          </select>
                          </td>


------------------------------------------------------------------
상단에는 아래와 같이 되어 있습니다.

// 나이 검색
if($mb_age){

switch($mb_age){

case "10" : $where .= " and mb_age between 10 and 19"; break;

case "20" : $where .= " and mb_age between 20 and 29"; break;

case "30" : $where .= " and mb_age between 30 and 39"; break;

case "40" : $where .= " and mb_age between 40 and 49"; break;

case "50" : $where .= " and mb_age between 50 and 59"; break;

case "60" : $where .= " and mb_age between 60 and 69"; break;

case "70" : $where .= " and mb_age between 70 and 79"; break;

case "80" : $where .= " and mb_age between 80 and 100"; break;

}

}


// 직업 검색
if($mb_job){

for($i=0; $i<count($mb_job); $i++){

if($mb_job[$i]){

switch($mb_job[$i]){

case "A1": $mb_job_code = "1"; break;
case "A2": $mb_job_code = "2"; break;
case "A3": $mb_job_code = "3"; break;
case "A4": $mb_job_code = "4"; break;
case "A5": $mb_job_code = "5"; break;
case "A6": $mb_job_code = "6"; break;
case "A7": $mb_job_code = "7"; break;
case "A8": $mb_job_code = "8"; break;
case "A9": $mb_job_code = "9"; break;
case "B1": $mb_job_code = "10"; break;
case "B2": $mb_job_code = "11"; break;
case "B3": $mb_job_code = "12"; break;
case "B4": $mb_job_code = "13"; break;
case "B5": $mb_job_code = "14"; break;
case "B6": $mb_job_code = "15"; break;
case "B7": $mb_job_code = "16"; break;
case "B8": $mb_job_code = "17"; break;
case "B9": $mb_job_code = "18"; break;
case "C1": $mb_job_code = "19"; break;
case "C2": $mb_job_code = "20"; break;
case "C3": $mb_job_code = "21"; break;
case "C4": $mb_job_code = "22"; break;
case "C5": $mb_job_code = "23"; break;
case "C6": $mb_job_code = "24"; break;
case "C7": $mb_job_code = "25"; break;
case "C8": $mb_job_code = "26"; break;
case "C9": $mb_job_code = "27"; break;
case "D1": $mb_job_code = ""; break;

}

$mb_job_res .= " mb_job = '".$mb_job_code."' or ";

} else {

$mb_job_res .= " mb_job = '' or ";

}

}

$mb_job_res = substr($mb_job_res, 0, strlen($mb_job_res) - 3);

//echo $mb_roundYoil_res."<br>";

$where .= " and (".$mb_job_res.") ";

}