I just learned the basics in sqlzoo and the rest on job, should be quick. The problem with learning is that students have no idea about real world requirements so they can only go for very generetic ones (average revenue last week). The key, IMO, is to connect with real world business requirements and start writing queries.

I worked as a BA and now work in BI so it's common for me to write long and complex queries. Most of them due to bad table structure but we can't do much so have to go around.