在PHP中从数据库中选择字符串

我有一个网页，你可以搜索数据库。用户可以搜索5个不同的字段，页面通过post发送输入。点击搜索时，某些字段可以为空。我可以使用一个很好的select语句，而不是一大堆if语句

$Country = $_POST['Country'];
$Gender = $_POST['Gender'];
$lastName = $_POST['lastName'];
$firstName = $_POST['firstName'];
$sport = $_POST['sport'];   

//selects sport and country
    if  (($lastName == null) && ($firstName == null) && ($Gender == null))
    {
    $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON          (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (name = '$Country') AND (sport ='$sport') ";
    }


  //selects country and gender and sport
  if (($lastName == null) && ($firstName == null))
    {

        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (name = '$Country') AND (gender ='$Gender') AND (sport = '$sport')";
    }


    //selects country and last and first name
    else if ($Gender == null)
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (name = '$Country') AND (firstName LIKE '%$firstName%') AND (lastName LIKE '%$lastName%') AND (sport = '$sport') ";
    }

    //selects sport, gender, last name and country 
    else if  ($firstName == null)
    {
    $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (name = '$Country') AND (sport ='$sport') AND (gender ='$Gender') AND (lastName LIKE '%$lastName%') ";
    }

    //selects sport, gender, first name and country 
    else if  ($lastName == null)
    {
    $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (name = '$Country') AND (sport ='$sport') AND (gender ='$Gender') AND (firstName LIKE '%$firstName%') ";
    }

    //selects just country
 if (($Gender == null) && ($lastName == null) && ($firstName == null) && ($sport == null))
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE  (name ='$Country') ";
    }


         //selects just sport
     else if (($lastName == null) && ($firstName == null) && ($Gender == null) && ($Country == 'country'))
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (sport = '$sport') ORDER BY sport ";
    }

         //selects just last name
     else if (($sport == null) && ($firstName == null) && ($Gender == null) && ($Country == 'country'))
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (lastName = '$lastName') ORDER BY sport ";
    }

    //selects gender and last name
    else if (($Country == 'country') && ($firstName == null))
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (lastName LIKE '%$lastName%') AND (gender LIKE '%$Gender%') AND (sport = '$sport') ";

    }


    //selects gender and first name
    else if (($Country == 'country') && ($lastName == null))
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (firstName LIKE '%$firstName%') AND (gender = '$Gender') AND (sport = '$sport')  ";

    }


    //selects country, sport and first name
    else if (($Gender == null) && ($lastName == null))
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (firstName LIKE '%$firstName%') AND (sport = '$sport') AND (name = '$Country') ";

    }


    //selects last name, sport and first name
    else if (($Gender == null) && ($Country == 'country'))
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (firstName LIKE '%$firstName%') AND (sport = '$sport') AND (lastName LIKE '%$lastName%') ";

    }
    // selects sport and gender
    else if (($Country == null) && ($lastName == null) && ($firstName == null)) 
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (gender = '%Gender%') AND (sport = '$sport') ";
    }

    // selects gender
    else if (($Country == null) && ($lastName == null) && ($firstName == null) && ($sport == null ) ) 
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (gender = '%Gender%')  ";
    }

    // selects country and last name 
    else if (($Gender == null) && ($firstName == null ) && ($sport == null))
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE  (name = '$Country') AND (lastName LIKE '%$lastName%') ";
    }


    // selects country and first name 
else  if (($Gender == null) && ($lastName == null ) && ($sport == null))
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE  (name = '$Country') AND (firstName LIKE '%$firstName%') ";
    }


        // selects all
    else if (($Gender == null) && ($firstName == null ) && ($sport == null) && ($lastName == null) && ($Country == 'country') )
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode )";
    }
    // selects if all feilds full
    else 
    {
        $selectString = "SELECT * FROM tblCountry JOIN tblAthletes ON (tblAthletes.countryCode = tblCountry.countryCode ) WHERE (name = '$Country') AND (gender ='$Gender') AND (lastName LIKE '%$lastName%') AND (firstName LIKE '%$firstName%') ORDER BY lastName ";
    }

    $result = mysql_query($selectString);


    while($row = mysql_fetch_assoc($result))
    {
    echo"<tr>";
        foreach($row as $index=>$value)
        {
            if(($index == 'flagImage')||($index == 'atheleteImage'))
            {
                //Gets images
                echo"<td><img title='competitor' alt='' src='images/$value' width='80' height='80'/></td>";
            }
         else
            {
                echo("<td>$value</td>");
            }
        }
    echo"</tr>";
    }
    echo"</table>";
    echo"</div>";

}

$Country=$\u POST['Country']；
$Gender=$_POST['Gender']；
$lastName=$\u POST['lastName']；
$firstName=$_POST['firstName']；
$sport=$_POST['sport']；
//选择运动和国家
如果（$lastName==null）&&（$firstName==null）&&（$Gender==null））
{
$selectString=“SELECT*FROM TBLCONTRY JOIN tblAthletes ON（tblAthletes.countryCode=TBLCONTRY.countryCode），其中（名称='$Country'）和（运动='$sport'）”；
}
//选择国家、性别和体育
如果（$lastName==null）和（$firstName==null））
{
$selectString=“SELECT*FROM TBLCONTRY JOIN TBLATTLETES ON（TBLATTLETES.countryCode=TBLCONTRY.countryCode），其中（名称=“$Country”）和（性别=“$gender”）以及（运动=“$sport”）；
}
//选择国家、姓氏和名字
else if（$Gender==null）
{
$selectString=“SELECT*FROM tblcontry JOIN tblAthletes ON（tblAthletes.countryCode=tblcontry.countryCode），其中（name='$Country'）和（firstName类似“%$firstName%”）和（lastName类似“%$lastName%”）和（sport='$sport'）；
}
//选择运动、性别、姓氏和国家
else if（$firstName==null）
{
$selectString=“SELECT*FROM tblcontry JOIN tblAthletes ON（tblAthletes.countryCode=tblcontry.countryCode），其中（name='$Country'）和（sport='$sport'）和（gender='$gender'）和（lastName类似“%$lastName%”）；
}
//选择运动、性别、名字和国家
else if（$lastName==null）
{
$selectString=“SELECT*FROM tblcontry JOIN tblAthletes ON（tblAthletes.countryCode=tblcontry.countryCode），其中（name='$Country'）和（sport='$sport'）和（gender='$gender'）和（firstName类似“%$firstName%”）；
}
//选择公正的国家
如果（$SEXT==null）&&（$lastName==null）&&（$firstName==null）&&（$sport==null））
{
$selectString=“SELECT*FROM TBLCONTRY JOIN tblAthletes ON（tblAthletes.countryCode=TBLCONTRY.countryCode），其中（name='$Country'）”；
}
//选择正义运动
如果（$lastName==null）&&（$firstName==null）&&（$Gender==null）&&（$Country==Country'），则为else
{
$selectString=“SELECT*FROM TBLCONTRY JOIN tblAthletes ON（tblAthletes.countryCode=TBLCONTRY.countryCode），其中（sport='$sport'）按运动排序”；
}
//只选择姓氏
如果（$sport==null）&&（$firstName==null）&&（$Gender==null）&&（$Country==Country'），则为else
{
$selectString=“SELECT*FROM TBLCONTRY JOIN tblAthletes ON（tblAthletes.countryCode=TBLCONTRY.countryCode），其中（lastName='$lastName'）按运动排序”；
}
//选择性别和姓氏
else if（（$Country=='Country'）&&（$firstName==null））
{
$selectString=“SELECT*FROM tblcontry JOIN tblAthletes ON（tblAthletes.countryCode=tblcontry.countryCode），其中（lastName类似“%$lastName%”）和（gender类似“%$gender%”）和（sport='$sport'）；
}
//选择性别和名字
else if（（$Country=='Country'）&&（$lastName==null））
{
$selectString=“SELECT*FROM tblcontry JOIN tblAthletes ON（tblAthletes.countryCode=tblcontry.countryCode），其中（firstName，如“%$firstName%”）和（gender=“$gender”）和（sport=“$sport”）；
}
//选择国家、运动和名字
else如果（$Gender==null）和（$lastName==null））
{
$selectString=“SELECT*FROM tblcontry JOIN tblAthletes ON（tblAthletes.countryCode=tblcontry.countryCode），其中（firstName类似“%$firstName%”）和（sport=“$sport”）和（name=“$Country”）；
}
//选择姓氏、运动和名字
如果（$SENDERY==null）和（$Country=='Country'），则为else
{
$selectString=“SELECT*FROM tblcontry JOIN tblAthletes ON（tblAthletes.countryCode=tblcontry.countryCode），其中（firstName像“%$firstName%”）和（sport像“%$lastName%”）和（lastName像“%$lastName%”）；
}
//选择运动和性别
else如果（$Country==null）和（$lastName==null）和（$firstName==null））
{
$selectString=“SELECT*FROM TBLCONTRY JOIN TBLATTLETES ON（TBLATTLETES.countryCode=TBLCONTRY.countryCode），其中（性别='%SENDER%'）和（运动='$sport'）”；
}
//选择性别
如果（$Country==null）&&（$lastName==null）&&（$firstName==null）&&（$sport==null））
{
$selectString=“SELECT*FROM TBLCONTRY JOIN tblAthletes ON（tblAthletes.countryCode=TBLCONTRY.countryCode），其中（性别=“%SEND性别%”）”；
}
//选择国家和姓氏
else如果（$Gender==null）和（$firstName==null）和（$sport==null））
{
$selectString=“SELECT*FROM tblcontry JOIN tblAthletes ON（tblAthletes.countryCode=tblcontry.countryCode），其中（name='$Country'）和（lastName类似“%$lastName%””）；
}
//选择国家和名字
else如果（$Gender==null）和（$lastName==null）和（$sport==null））
{
$selectString=“SELECT*FROM TBLCONTRY JOIN tblAthletes ON（tblAthletes.countryCode=TBLCONTRY.countryCode），其中（name='$Country'）和（firstName类似“%$firstName%””）；
}
//选择全部
如果（$Gender==null）和（$firstName==null）和（$sport==null）和（$lastName==null）和（$Country==Country'），则为else
{
$selectString=“选择*从TBLContry加入tblAthletes ON（tblAthletes.countryCode=TBLContry.countryCode）”；
}
//选择所有字段是否已满
其他的
{
$selectString=“SELECT*FROM TBLCONTRY JOIN tblAthletes ON（tblAthletes.countryCode=TBLCONTRY.countryCode），其中（名称=‘$Country’）和（性别=‘$GENDERY’）
SELECT
    *
FROM
    your_table_here
WHERE
        (('' = :country) OR country = :country)
    AND (('' = :gender) OR gender = :gender)
    AND (('' = :lastName) OR lastName = :lastName)
    AND (('' = :firstName) OR firstName = :firstName)
    AND (('' = :sport) OR sport = :sport)
;

SELECT
    *
FROM
    your_table_here
WHERE
        (IS NULL(:country) OR country = :country)
    AND (IS NULL(:gender) OR gender = :gender)
    AND (IS NULL(:lastName) OR lastName = :lastName)
    AND (IS NULL(:firstName) OR firstName = :firstName)
    AND (IS NULL(:sport) OR sport = :sport)

SELECT `Country`, `Gender`, `lastName`, `firstName`, `sport` FROM TABLE_NAME WHERE Country='$country'...etc