Wordpress featured image / post thumbnail not showing - wordpress

Question

I'm trying to show the featured image or the post thumbnail in every list items < li >. Here's the code:
<div class="row headline">
<ul class="col-md-12 col-sm-12">
<?php
$sql_submenu_produk_sql = $wpdb->get_results("
SELECT * from $wpdb->posts
WHERE post_parent=52 AND post_type='page' AND post_status='publish' group by ID;
");
?>
<?php foreach ($sql_submenu_produk_sql as $row_submenu_sql) { ?>
<li>
<div class="col-md-2 col-xs-12 bordered" style="min-height: 100px;">
<a href="" data-featherlight="image">
<?php echo get_the_post_thumbnail($post->ID); ?>
</a>
</div>
<div class="col-md-10 col-xs-12">
<h2 style="font-weight: normal;"><?php echo $row_submenu_sql->post_title ?></h2>
<p style="padding-bottom: 15px;"><?php echo $row_submenu_sql->post_content ?></p>
</div>
</li>
<?php } ;?>
</ul>
</div>
but, it's not showing after all while on the other page shows. Please help.

Answer 1

Should you have get_the_post_thumbnail($row_submenu_sql->ID); and not $row_submenu_sql->ID $post->ID is not defined in that loop.
<div class="row headline">
<ul class="col-md-12 col-sm-12">
<?php
$sql_submenu_produk_sql = $wpdb->get_results("
SELECT * from $wpdb->posts
WHERE post_parent=52 AND post_type='page' AND post_status='publish' group by ID;
");
?>
<?php foreach ($sql_submenu_produk_sql as $row_submenu_sql) { ?>
<li>
<div class="col-md-2 col-xs-12 bordered" style="min-height: 100px;">
<a href="" data-featherlight="image">
//UPDATE THIS LINE LIKE BELOW
<?php echo get_the_post_thumbnail($row_submenu_sql->ID); ?>
</a>
</div>
<div class="col-md-10 col-xs-12">
<h2 style="font-weight: normal;"><?php echo $row_submenu_sql->post_title ?></h2>
<p style="padding-bottom: 15px;"><?php echo $row_submenu_sql->post_content ?></p>
</div>
</li>
<?php } ;?>
</ul>
</div>