How to fetch data from mysql database while scrolling

step 1] Take a help of little bit jquery as....

var countScroll= 0;
$(window).scroll(function()
{
      if  ($(window).scrollTop() == $(document).height() - $(window).height())
      {
         loadData();
      }
      countScroll++;
});

step 2] Take the help of ajax to call the loadData() function

function loadData()
{
    var xmlhttp;
    if (window.XMLHttpRequest)
    {// code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    }
    else
    {// code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange=function()
    {
        if (xmlhttp.readyState==4 && xmlhttp.status==200)
        {               
        var res = xmlhttp.responseText;
        document.getElementById("respDiv").innerHTML=res;           
            }
    }
    xmlhttp.open("GET","loadPageData.php?loadLimit="+countScroll,true);
        xmlhttp.send();
   }

step 3] your php page i.e. loadPageData.php is as...

    $loadLimit= $_GET['loadLimit'];
    $result = mysql_query("SELECT * FROM tableName limit $loadLimit");
    if(mysql_num_rows($result)>0)
    {
        while($row = mysql_fetch_array($result))
        {
        echo $yourVariable= $row['fieldName'];
          }
    }

Your question is generic and difficult to answer with snippet of code.
This could be a solution in pseudo-language

• Fetch a default number of element (say 50) by doing a query like this SELECT * FROM yourTable WHERE ..... ORDER BY .... LIMIT 50
• Use jQuery to detect scroll event
• Make an AJAX call once you detect scroll event
• Make a new query where you fetch 50 elements again, but this time take into account that you have scrolled (so, add a delta or offset)
• Render elements returned from AJAX

Use jQuery (or your favourite library) to detect when the user gets to the end of the page, then just trigger an AJAX request to the server for the next N results.

For example:

JavaScript part:

var n = 0;

$(document).ready(function() {
    $(window).scroll(function() {
        if ($('body').height() <= ($(window).height() + $(window).scrollTop())) {
            $.ajax({
              url: "my_page.php",
                              data: {'n' : n},
              context: document.body
            }).done(function(data) { 
              n += 1;
              console.log(data); // <--- do stuff with received data here
            });
        }
    });
});

PHP part:

$n = $_POST['n'];
$n = 10 * $n;
$sql = "SELECT * FROM mytable LIMIT $n, 10";  // get 10 new entries each request.