Retroactive addition of Featured Image (without using first post image)

I am moving a 1200 post strong WordPress site from a WordPress.com to our own host. As you might suspect, it does not go too smoothly.

I’ve managed to get everything moved over except one rather important thing: the featured images!

For some reason, the importer could not correctly import any featured images and those are rather important for the theme we are using. I am fine with having a single picture for all posts for now, but I do not want to go into 1200 different posts and manually set every featured image.

I have seen a LOT of threads out there recommending different plugins that drag the first image from the post into a featured image, but that won’t work, since for the majority of the posts, the featured image is the ONLY image.

A few other plugins will generate a new image or set a standard image, but those only work if you go into the edit window and manually press update and if I have to do that, I might as well manually set the featured image.

I’ve tried to look for an SQL code to fix it, but I can’t find any and I don’t have the knowledge to make one that complicated (I can’t figure out how to sort all articles that DON’T have a featured image, since the meta for each post has a lot more rows than that).

Could anyone help me find an automated way to fill all featured images retroactively, or do I have to do it manually?