选择带有范围的下拉列表，然后在数据库中搜索

I'm new in this field so please don't be harsh on me. It's looks simple yet I can't figured it out. So, I have dropdown select, like:

<select name="mode" id="mode">
    <option value="None">All</option>
    <option value="300-400">Inside</option>
    <option value="200-299">Outside</option>
</select>

Then I currently have this

$cMode=($mode == "None") ? '"%"' : $mode;

SELECT * FROM sections WHERE code LIKE '.$cMode.'

How can I change the query so if I select Inside to search between values 300 and 400?

You can use if-else with explode() and list()

if($mode == "None"){
    $cMode = '"%"';
    SELECT * FROM sections WHERE code LIKE '.$cMode.'
}else{
    list($start,$eand) = explode('-',$mode);
    SELECT * FROM sections WHERE code BETWEEN $start AND $end
}

As you asked in the comment:-

  $cMode ='';
   if($mode == "None"){
     $cMode = 'LIKE "%"';
   }else{
     list($start,$eand) = explode('-',$mode);
     $cMode ="BETWEEN $start AND $end";
   }

$query = "SELECT * FROM sections WHERE code $cMode";

You can make $cMode dynamically by using if and else condition as:

   $cMode='';
   if($mode == "None"){
     $cMode = 'LIKE "%"';
   }else{
     $arr = explode('-',$mode);
     $cMode ="BETWEEN $arr[0] AND $arr[1]";
   }

$query = "SELECT * FROM sections WHERE code $cMode";

For more details SQL BETWEEN Operator

I haven't quite understood what you were asking for, this is an example of how I print out data from rows with filters

<form action="PAGE.php" method="POST" id="users_selection">
    <select name="mode" id="mode">
        <option value="None">All</option>
        <option value="300-400">Inside</option>
        <option value="200-299">Outside</option>
    </select>
    <input type="submit" value="choose" name="choosing_user">
</form>
<?php
    $servername = "";
    $susername = "";
    $spassword = "";
    $dbname = "";
    $conn = mysqli_connect($servername, $susername, $spassword, $dbname);
    $mode = $_POST['mode'];
    $sql = "SELECT * FROM sections";
    $result = $conn->query($sql);
    if ($result->num_rows > 0) {
        while($row = $result->fetch_assoc()) {
            $MYSQL_data = $row['data'];
            if($mode=="None"){
                echo '<p>'.$MYSQL_data.'</p>';
            }else if($mode=="300-400"){
                if($MYSQL_data>299 && $MYSQL_data <401){
                    echo '<p>'.$MYSQL_data.'</p>';
                }
            }else if($mode=="200-299"){
                if($MYSQL_data>199 && $MYSQL_data <300){
                    echo '<p>'.$MYSQL_data.'</p>';
                }
            }
        }
    }
?>