Itzik Ben-Gan lays out some fundamentals:
When people start learning a new field, for example T-SQL, it’s tempting to spend very little time trying to understand the fundamentals of the field so that you can quickly get to the advanced parts. You might think that you already understand what certain ideas, concepts and terms mean, so you don’t necessarily see the value in dwelling on them. That’s often the case with newcomers to T-SQL, especially because soon after you start learning the language, you can already write queries that return results, giving you a false impression that it’s a simple or easy language. However, without a good understanding of the foundations and roots of the language, you’re bound to end up writing code that doesn’t mean what you think it means. To be able to write robust and correct T-SQL code, you really want to spend a lot of energy on making sure that you have an in-depth understanding of the fundamentals.
No matter your knowledge level, there’s a really good chance you’ll learn at least one new thing in this article.