C != Mastering Low Level Computing
Different subjects. C is just a means to an end, but other languages (e.g. assembly, C++) can also work towards this end.
If you want to learn low level computing, you should take a computer architecture course, and even work through assembly.
C++ is not just "helpful for professional programmers to build large projects", it's a fundamental improvement to every aspect of C, as well. Again, it's up to you, but you seem to have some misconceptions about C++. C++ ranges all the way from low level to high level. You can master low-level programming in C++ too, if you so choose. You just need to stay away from higher-level abstractions. "a lot of syntax I find unnecessary" is a very weak argument. It seems to me more likely that you aren't familiar with the syntax, so you just can't be bothered to learn the syntax regardless if it helps you achieve your goals or not.
And SQL is unrelated to C and C++. I also use SQL, but that that project which uses the database, I use C#, since C just doesn't cut it (for me). So, it depends on the environment and requirements, really. Also learning C will NOT help make you a better SQL programmer. If you want to learn SQL, go learn SQL, not C. Way different areas.
You also seem to be under the impression that, if you learn C, you somehow magically are able to gain significant understanding or master over other fields or programming tasks because C can do them. That's not true. If you're going to use heavy object orientation, for example, you're not going to use C. You're going to use C#, C++ or Java. That's because they are better at it that C, even if C can theoretically DO it. But you still have to learn those languages and how to object orientation in an optimal way too. Learning C doesn't teach you that. Similarly, mastering C won't teach you how to use SQL efficiently or even understand it very well. It won't make you a better SQL programmer. What it WILL do is perhaps make you a better C programmer. But that's it.
If you want to master a subject, then study that subject, not its underpinnings. SQL and C are completely different.