If you want to set a featured image on all wordpress posts that dont already have a featured image set, then this mysql query can help:
INSERT INTO wp_postmeta (post_id,meta_key,meta_value) SELECT p.ID, '_thumbnail_id', id_of_image_from_upload_table FROM wp_posts p LEFT JOIN wp_postmeta pm ON p.ID = pm.post_id AND pm.meta_key = '_thumbnail_id' WHERE p.post_type IN ('post') AND p.post_status IN ('publish') AND pm.meta_key IS NULL LIMIT 0 , 100000;
# please note that this query will fail; you have to replace ‘id_of_image_from_upload_table’ with the id of the image you uploaded.
To find that ID, upload an image to the media library, then run this query:
SELECT ID FROM wp_posts WHERE post_type = 'attachment' ORDER BY ID DESC LIMIT 1;
You could join it all together and run it in a single query if you like..
Finally, update the postmeta table for entries who have a thumbnail image, but a null value:
update wp_postmeta set meta_value=id_of_image_from_upload_table where meta_key = '_thumbnail_id' and meta_value is null;