UPDATE wp_options SET option_value = replace(option_value, 'www.mydomain.com','www.newdomain.com') ;
UPDATE wp_posts SET post_content = replace(post_content, 'www.mydomain.com','www.newdomain.com') ;
UPDATE wp_comments SET comment_content = replace(comment_content, 'www.mydomain.com', 'www.newdomain.com') ;
UPDATE wp_comments SET comment_author_url = replace(comment_author_url, 'www.mydomain.com', 'www.newdomain.com') ;