Ask HN: How to improve my SQL skills?

https://sqlbolt.com is a really nice interactive site that makes you pass practice queries before continuing. Gives you a sense of accomplishment when you finally figure it out.

I also really like https://www.w3schools.com/sql/trysql.asp?filename=trysql_op_... for practice but be warned it runs SQLite which can be confusing if you're more used to built in functions from other things like SQL server

http://sqlfiddle.com is another nice query building site. It gives you the option of working with different engines.