I understand that HTML5 "localStorage" is a key:value store but I am wondering if there is a Javascript library available that offers a more SQL-ish API?
HTML5 local database storage comes with a SQL interface by default, if I'm not mistaken
Here is a Webkit post with some examples: http://webkit.org/blog/126/webkit-does-html5-client-side-database-storage/
Currently, Chrome forces you to use Gears, which is slightly different, but still SQL-based. Future versions of Chrome will follow the HTML5 spec, however.
Check out Will HTML5 be SQL-free? and DOM Storage: a Cure for the Common Cookie for some links and opinions.
W3C Database specification says:
User agents must implement the SQL dialect supported by Sqlite 3.6.19.
As of now, at least Google Chrome supports SQL dialect. I have checked myself.
You should use HTML5 database storage (it supports SQL through transactions). a tutorial here: http://www.html5rocks.com/tutorials/webdatabase/todo/