If you have imported a bunch of nodes from another source (perhaps using the Blogger Importer module) and the author has defaulted to Anonymous, you can set all the nodes to a specific author with a MySQL query.
UPDATE `node` SET uid = "1"
Use at your own risk. Important: This will set all nodes to author uid 1. If you have multiple uids you need to keep track of, you’ll have to filter them with a SELECT statement first.