So I'm a bit of an expert in SQL Server and in my not so humble opinion there is one question that is the most telling when it comes to SQL Server expertise: W

So I'm a bit of an expert in SQL Server and in my not so humble opinion there is one question that is the most telling when it comes to SQL Server expertise: What is a join, what are the different algorithms to implement a join, and when is each one suitable or not suitable. That's it. Someone who answers this question completely can be trusted to learn the rest of the nitty-gritty from the docs, and someone who can't answer it will most certainly screw up your product by implementing a join manually by using a cursor.