Get a WordPress featured image by Post ID, replace 12345 with your post ID.

wpFeaturedImage.sql

SELECT
	`wp_postmeta_1`.`meta_value` 
FROM
	`wp_postmeta`
		INNER JOIN `wp_postmeta` `wp_postmeta_1` 
		ON `wp_postmeta`.`meta_value` = `wp_postmeta_1`.`post_id` 
WHERE
	(`wp_postmeta`.`post_id` = 12345) AND
	(`wp_postmeta`.`meta_key` = '_thumbnail_id') AND
	(`wp_postmeta_1`.`meta_key` = '_wp_attached_file')