When asking database questions, new users often forget to tag their question both with sql and any of mysql, oracle, postgres, sqlserver, or whatever. People then always need to ask "What database is this about?", e.g:
Often, some clever users then start to guess SQL dialects before the actual dialect is supplied, which will then render their answers invalid.
Very few questions are really SQL-dialect-agnostic, and thus "pure" SQL questions. Wouldn't it be interesting if a warning would pop up next to the "Post Your Question" button or next to the tags field, indicating that the current set of tags might not be complete or precise enough? I would imagine that other "general" tags with related "specific" tags may exist. In this example, if the sql tag is present, any of the related mysql, oracle, postgres, sqlserver, or whatever tags might be useful to add.
In fact, this goes along the lines of "Is your tag-set complete? Because users who tagged their questions with sql often also tagged their questions with..."
A similar request has been made before, although I do not think that tags need that much structuring: A proposal for tag hierarchy on Stack Exchange sites