You may want to cover some of the core concepts from SQL. I have seen tremendous people working on SQL not just developers but even Business Analysts and some Marketing teams. Those are the kind of people who need help running different kinds of queries. You might be interested in writing the use case based queries.
Blogging about core concepts seems to me too much like writing a tutorial - not so sure a blog is a good platform for that. I'll be happy to write about specific queries for specific use cases, though.
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