Should SQL challenges (performance-related or otherwise) be considered on topic for this site? By "SQL challenge" I mean attempting to solve a problem using SQL that may not have any immediate, real world application. In some cases it could be obvious that SQL is the wrong choice of language for the problem. I think that a strict interpretation of the rules would consider these to be off topic, but I find them to be a fun way to be exposed to new ways of solving problems and to develop technical skills.
To try to better explain what I mean here, I would define a challenge as writing code for the sake of writing code. I know the example questions below weren't closed, but I'm asking for the sake of future questions that might be asked. For example, consider dynamic data masking in SQL Server. I think that asking for queries which can quickly unmask data of a certain type would be an interesting exercise. Do I have a practical need for such an answer? No, I'm not a malicious hacker. Could I create a fake scenario for such a question? Sure, I could say something like "My boss is convinced that dynamic data masking will protect our data. Is it possible to write a very efficient data that unmasks all of the data?" Is that scenario necessary for it to be on topic?
Here are some examples of what I would describe as SQL challenges: