SQL command to delete all users with karma 1

I have more than 2000 user accounts which need to be deleted. In the /admin/ page I cannot filter these users out. Is there perhaps an SQL command that allows me to delete all the users with karma 1 (or users without any posts). To be sure -- because I am not very technical -- what is an SQL to backup the database first. And what would be to a SQL command that lists all the users satisfying my criteria. Just want to be sure that I don't delete all users. Thanks :)