I've seen it happen a bunch. Since StackOverflow is a Q&A format, if someone knows that many people will ask a question they know the answer to, they will write

I've seen it happen a bunch. Since StackOverflow is a Q&A format, if someone knows that many people will ask a question they know the answer to, they will write both. Preemptive questioning perhaps! He also wrote the other question that he links to, How to index a database column, with the request to get answers for each major type of database. So he's asking not so much to learn the answer but to provide a place for others to provide a catalog of answers. This is different of course from the case where someone asks a genuine question then comes back and writes an answer to themselves when they have learned it. This happens a lot too.