There are a few options. If you've done the basics through Codecademy, then you're ready to move on to the specifics of a platform (such as PostgreSQL as you've mentioned). Though this can equally be a minefield.

The reporting engines/formats differ from platform to platform. As does SQL believe it or not, in the sense each of the mainstream vendors adds their own specific bits to the standard. They are all compliant for the basics though. All of them will support the latest version of SQL (ANSI standard 2011), but PL/SQL is different to Transact-SQL in the way it uses cursors.

In addition, each has an even bigger variation when it comes to multidimensional queries such as MDX, since there is no standard for MDX and some vendors use their own expressions and languages to carry out such queries.

I can certainly help with tutoring, having been working in SQL since 1997 (and still do), although I think it's worth focussing on getting a little better at the specifics of the SQL vendor extensions you want to use, including their Cube, Pivot and Grouping functions as well as more complex joins, as they'll be the baseline skills for future reporting exercises, including analytical processing.