Based on the StackOverflow data dump, it seems that S.O. represents questions and answers as a single table - Posts.
However, a question has a title, a body and tags associated with while an answer only has a body. To me, at least, this indicates that they are distinct enough that they should be separate tables.
Besides, I don't like having to write "and type='question'" in my SQL.
Are these valid reasons?
Or is there a good reason for putting questions and answers into the same table?
I would separate them on principle, just because they are different beasts. Question have titles (as you said), tags, favorite markers and (presumably) are subject to searches for attempted duplicate detection.
That would seem to me to make them different enough to warrant a separate table.
However, we don't know how SO stores them in the database, you've only seen the export into the data dump - it may be that the export functionality combines questions and answers into posts.
It may also be that the information common to questions and answers are stored in a single table and the question-specific extras stored in another table. Short of asking the SO developers, I can't think of any way to confirm this.
Questions and answers have a lot in common -- author, date, comments, &c. Separating the table (since SQL schemas typically don't support inheritance) means a lot of duplication (the comments table will likely also have to be split, or have a goofy design with two foreign keys, one to the Q table and one to the A table, of which exactly one is to be non-NULL).
Yes, there are distinctions too between Q & A, and advantages the other 'way round, too, as you point out. "You pays your money, you takes your choices"!-)
It is better to have questions and Answers in seperate table. You can map them using the Question ID (Ex: 985113 in this question)
There is an option to close questions (>3k rep) in SO and one of the reason is duplicate question. We have to enter the question ID or part of question. Remeber searchinif both answers and questions are in the same table.
Actually I think we've heard enough hints on the podcast to suggest that they are stored in the same table - also it looks like the ID numbers for questions and answers do not overlap. Maybe they did it for performance reasons? It will be possible for example to fill the data for a page like this one in a single scan of the posts table, rather than one scan of questions and one of answers.