That thing "Whatever Mike" that you are asking for, woul dbe untrue if we write it, because you explained 'close to nothing', so there's not much to understand really.
For example/:
What do you mean by "to use"? If you want whatever is held in that postmeta to be used and filled in as featured image for the post then you should have said what it is that this postmeta value is - file location, URI, atatchment id?
If the value is not an ID of an image already added to th emedia library, then this is no SQL job at all. The fetured image is kept as _thumbnail_id postmeta, meaning the image has to be added to media library first and then assign with the post.
If by 'to use' you mean someting else then, apparently you have to specify what that is, so we can address the matter appropriately.
Regards,
Dobri