What are some good tips/resources for improving my SQL prowess?

I've been working in a Microsoft dev shop for close to three years, but feel like I stopped learning after a month or two. Entering with no real-world SQL experience, I've learned on the job how to get things done here for any task I'm assigned, but sadly there is no code review in this company and I don't know that I'm doing a good job from a technical standpoint.

Performance is now a rising topic of discussion as our platform is scaling up, so I want to be ready for any optimization tasks that may be available in the near future. As the majority of our business logic lives in SQL Server, I think that learning SQL (specifically T-SQL / SQL Server) more properly would be of great immediate value to the company and my own career alike.

Long backstory, shorter question: Could anyone please recommend some resources to learn SQL, maybe geared toward those who think they know what they're doing and are not beginners, but don't know much low-level detail on how their queries are actually running?

Thanks, y'all ✌