将带有数据值的列表中的值传递给mysql查询

I have a list

<ul id="my_ul">
<li data-value="1"><a href="test.php">111</a></li>
<li data-value="2"><a href="test.php">112</a></li>
<li data-value="3"><a href="test.php">113</a></li>
</ul>

I tried to pass the data-value on click event to MySQL query. I tried it with jQuery but when i var_dump the result its NULL.

Here is how i am getting the value

$('#my_ul').on("click", "li", function(){
   $.ajax("fetch_test.php",{method:"post", data:{val:$(this).attr("data-value")}}); 
});

and here is the fetch_test.php file code :

function showValue(){
include_once "connect.php";
$test=$_SESSION['val'];
$query="SELECT * FROM product INNER JOIN smetka on smetka.id=product.smetka_id WHERE product.smetka_id=$test";
$result=mysqli_query($conn,$query);
    $alert="";
        while($row=mysqli_fetch_array($result)){
            $alert .="<li data-value='$row[id]'><a href='test.php'>$row[name]";

            $alert .= "</a></li>";
        }
    return $alert;
}

How can i pass the data-value from the li?

You should bind the event handler with anchor element and use event.preventDefault() to cancel its the default action. You can use .closest() to traverse up to li element.

$('#my_ul').on("click", "li a", function(e) {
  //Cancels default action of anchor
  e.preventDefault();
  var value = $(e.target).closest('li').data("value");
  console.log(value);
  $.ajax({url: "fetch_test.php", method:"post", data:{val:value}});
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<ul id="my_ul">
  <li data-value="1"><a href="test.php">111</a></li>
  <li data-value="2"><a href="test.php">112</a></li>
  <li data-value="3"><a href="test.php">113</a></li>
</ul>

</div>

Use event.preventDefault() to prevent redirection to test.php

Instead of

$('#my_ul').on("click", "li", function(){
   $.ajax("fetch_test.php",{method:"post", data:{val:$(this).attr("data-value")}}); 
});

Use

$('#my_ul').on("click", "li", function(event){
event.preventDefault();
   $.ajax("fetch_test.php",{method:"post", data:{val:$(this).attr("data-value")}}); 
});