I have a simple litmus test that I use to determine if the question belongs here or not. That test is basically this: if you take out everything specifically related to Salesforce, does the answer change at all, or does the question lose all purpose? If the answer, either way, is yes, then the question is on-topic here, otherwise it is not. I have sometimes broken this rule for ultra-simple questions that I felt would be ripped to shreds on SO, but these are rare exceptions.
Marketing Cloud SQL is itself a standard dialect of SQL, but a non-Salesforce-expert wouldn't know what to do with a _bounce or _click table without clarification or documentation, while an MC expert would likely be able to answer the question easily. This is what makes it on-topic, in my opinion. The subject matter involves tables unique to a Salesforce product, and answers given in a non-Salesforce context would likely be incorrect or misleading.
I personally don't have enough experience in MC to answer most questions, even though I do have a pretty solid grip on SQL. This is because MC does things differently in subtle ways than a generic SQL server (e.g. MySQL, Postgres, etc), and so needs to be handled by an expert in the product. I can generally recognize the difference between a MC question and a generic SQL question despite my lack of knowledge with MC; if I can answer it without looking at the MC docs, it's probably off-topic for this network.
After a cursory review, I feel like all of those questions could be considered on-topic, even the one that was closed off-topic and deleted by the system. They pass the test of needing a Salesforce context to make sense of the question.