SQLZoo
Regarding my old school SQL, I found this site that covers new school SQL (or, in my case, SQL that isn’t at least ten years out of date):
I think the original name for the site was A Gentle Introduction to SQL. Conveniently, it shows SQL variants across DB engines, such as MySQL, Oracle, SQL Server and Postgress. This is as much a syntactical reference as the official MySQL documentation. A quick glance through the site, and I don’t see such exotic structures as correlated subqueries, but I do see string manipulation functions, which is perhaps more what I’m after.