如何制作一个仅显示MySQL最后20个值的php动态表？

Question

I have made a php table that displays data from MySQL table. This is working perfectly. However, the MySQL table keeps getting updated and so my php table gets really long and big. I just want to keep it short, so I just want it to display the last 20 items on the table if possible.

Also the table doesn't update dynamically, to see the new updates you have to keep refreshing the page and I would like it if it could keep updating on its own . Here is the page of how the (long) table looks.

Here is my php code as of now:

<?php
    $con = mysql_connect("$host","$username","$pass"); 
    if (!$con)
    {
    die('Could not connect: ' . mysql_error());
    }
    mysql_select_db("$db_name", $con); //database name
    $result = mysql_query("SELECT * FROM $db_table"); //table

    echo "<table cellpadding='0' cellspacing='0' class='ver-minimalist'>
    <tr>
    <th>Photoresistor</th>
    <th>Accelerometer</th>
    </tr>";

    while($row = mysql_fetch_array($result))
    {
    echo "<tr>";
    echo "<td>" . $row['Photoresistor'] . "</td>";
    echo "<td>" . $row['Accelerometer'] . "</td>";
    echo "</tr>";
    }
    echo "</table>";

    mysql_close($con);
    ?>

I know I'm going to have to make a loop but I'm not sure which one would be the best option. Please, I would really appreciate the help.

The MySQL table does have an ID -primary key if that helps.

Answer 1

Make your query this:

SELECT * FROM $db_table ORDER BY id DESC LIMIT 20

Limit allows you to limit the amount of results you get from a query. By sorting by ID you can get the last 20 results in your table.