Thanks, all! Got it working.

Here's what happened: somewhere along the way, it looks like an upgrade script wasn't run. I looked at my blog posts to figure out which version of s9y I'd been using (initially, 1.4). That's why the following line didn't work!
mysql> UPDATE serendipity_authors SET ...

I've been locked out of my personal blog for quite some time and want to fix that. I just upgraded the software (admin page says "Powered by Serendipity 1.5.5 and PHP 5.2.17") and all looks good on main page (except for plugins, broken by an upgrade?)

I can access the database.
mysql> select ...