The most annoying thing I hear often is that if you use Hibernate you don't need to know SQL. Last time I checked, Hibernate's manual had something like a thousand pages. It's better to just learn SQL.
W3Schools has a good SQL tutorial I recommend to new engineers. After that just try to use it and use stack overflow when stumped. Easy things are easy, but some harder things can be difficult for some to grok.
1. Have a database to query (or think of some project that is basically a frontend on a database and which you can give a reasonable amount of data)
2. Come up with questions for the database you want to answer
3. Look at the SQLite documentation. They have fantastic diagrams for the syntax of a sql query.
4. Write down the sql for your query (or work out what you can’t express and see if you can search on the internet to find out how to express it)
5. Look into relational algebra and try to work out how it corresponds to sql databases.
6. Repeat steps 2-4
7. Understand what different types of joins there are
8. Repeat steps 2-4
9. Read about more exotic things like nested queries, window functions, recursive queries, and so on (just look at the features of your database of choice and see what you can learn about). These might come in handy for future questions
10. Repeat steps 2-4 (repeatedly)
If you don’t know which database to use then start with SQLite. It’s basically fine for anything simple and manything complicated.