SQL qiery to get all posts without featured image

I would use WP_Query:

$args = array(
	'post_type' => 'post',
	'post_status' => 'publish',
	'meta_query' => array(
		array(
			'key' => '_thumbnail_id',
			'value' => '?',
			'compare' => 'NOT EXISTS'
		)
	),
);
$the_query = new WP_Query( $args );

If you really need the SQL query string, the above query more or less generates this one:

SELECT wp_posts.* FROM wp_posts
   LEFT JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id
      AND wp_postmeta.meta_key = '_thumbnail_id')
   WHERE wp_posts.post_type = 'post'
      AND (wp_posts.post_status = 'publish')
      AND ( wp_postmeta.post_id IS NULL )
   ORDER BY wp_posts.post_date DESC

Worked flawlessly, thanks so much!

How do we run the code?

To use this code, refer to Class Reference/WP Query for use of the WP_Query class. The first “standard loop” example would take the above $args array verbatim to run the query as discussed here. This code typically would be on a custom template of some sort.