Ask HN: Resources to relearn my undergraduate courses?

As strange as it may sound, I graduated with a CS degree and didn't put in much effort during my time in college so I'd like to go over my undergraduate course material again. I need good resources to learn the following topics at an undergraduate/graduate level :

- Databases and SQL

- Compilers, formal languages and automata theory

- Operating systems

- Computer networking

If you have recommendations on books or courses online that could help me go over these topics, that would be very helpful.