[ticmasta]

Not required, but knowing a little relational algebra helps queries make a lot more sense. A lot of the features like constraints and indices are very thin veneer over the underlying concepts.

At the other end of the spectrum, once you can write a few basic queries check out something like SQL Murder Mystery https://mystery.knightlab.com/

This one is super-fun and lets you practice some basic to low-intermediate skills.

[fizixer]

Fun fact: You can do relational algebra on plain text data (e.g., csv, as well as output of commands like ls, etc.) using only shell + piping + well-known shell tools.