I always wanted do invest my 20 hrs into SQL. I've already put 10 with a data analyst course so i know what a sqlite , select and joins are. I've created a not trivial query once. Is this OK for learn some more ?
Sure thing. I think learning SQL is never wasted time. Try looking into SQL functions & TRIGGERs, LATERAL JOINs, ROLLUP & CUBE. If you have even more time, check out the different things an SQL server can do when you DROP a row that contains references, how you define CONSTRAINTs, and some particular features that only some subset of SQL dialects have, such as LISTEN/NOTIFY for PostgreSQL.
If you're a developer, I would look into CONSTRAINTS way before getting to anything like this. Learning how to model your constraints correctly in the database (preventing duplicates, etc) is one of the most important bits of schema design because it will save you tons of code and likely a bunch of race conditions.