There is already [a fine answer](https://meta.mathoverflow.net/a/5858) - this is just a minor addition. Some people might prefer Markdown for the posts rather than HTML. The other query uses `Body` from the `Posts` table - this contains HTML. Markdown can be obtained using the `PostHistory` table.

 * Here is a query which [returns Markdown for a single post](https://data.stackexchange.com/mathoverflow/query/1816895/markdown-for-a-specific-post?id=23478).
 * Here you can get Markdown for posts of a specific user: [only questions](https://data.stackexchange.com/mathoverflow/query/1816894/markdown-for-posts-by-a-specific-user?userid=8250&types=1), [only answers](https://data.stackexchange.com/mathoverflow/query/1816894/markdown-for-posts-by-a-specific-user?userid=8250&types=2) and [both](https://data.stackexchange.com/mathoverflow/query/1816894/markdown-for-posts-by-a-specific-user?userid=8250&types=1,2).
 * A similar query for posts from a deleted user: [only questions](https://data.stackexchange.com/mathoverflow/query/1816896/markdown-for-posts-by-a-deleted-user?userid=9072&types=1), [only answers](https://data.stackexchange.com/mathoverflow/query/1816896/markdown-for-posts-by-a-deleted-user?userid=9072&types=2) and [both](https://data.stackexchange.com/mathoverflow/query/1816896/markdown-for-posts-by-a-deleted-user?userid=9072&types=1,2).
 
I will repeat a few of the usual caveats:
 * [SEDE](https://meta.mathoverflow.net/tags/data-explorer/info) is only updated once a week - so you won't get very recent posts.
 * You don't get the deleted posts in this way. (There is a table `PostsWithDeleted` which contains *some* data about deleted posts - but neither body nor the post owner. So it cannot be used for the purposes discussed here.)
 * The results of a query are limited to 50000 rows - but so far even the most prolific posters on MO are [below 50k posts](https://data.stackexchange.com/mathoverflow/query/1580473/users-with-most-posts-questions-answers) (or [50k comments](https://data.stackexchange.com/mathoverflow/query/1798655/users-with-most-comments)) - so this won't be problem for MO in the near future.

Naturally, it is possible to do much more - if you have a look at
[rene's answer to Dump of my own Stack Exchange content](https://meta.stackexchange.com/a/269951), you can see that there is a query which collects contributions over all sites, creates a HTML file consisting of all results. And also an alternative query, which returns Markdown - again for posts from all sites in the network.