Querying Database with wpdb

I tested your all attempts in a shortcode:

function add_test_shortcode() {
  global $wpdb;
  global $post;
  $post_ID = $post->ID;
  $wpdb->show_errors = true;
  $result = $wpdb->get_var('SELECT meta_value FROM '.$wpdb->postmeta.' WHERE meta_key="my_test" AND post_id=' . $post_ID);

  return $result;
}
add_shortcode( 'test', 'add_test_shortcode' );

It worked perfectly for me, so there might something from your own code:

1. Check for $post_ID

Make sure your $post_ID is valid, get_the_ID() might be a better replacement in so many cases than global $post;.

2. Test your generated query

You can get your last executed query using $wpdb->last_query, execute it in phpMyAdmin or any other software and check that there is really a result.

3. Look if there’s any error

You can get the latest error statement using $wpdb->last_error.

My advice to you during WordPress development is that always keep WP_DEBUG on.

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