Do not confuse databases with web pages.
A database is just a collection of data, whose access is available only to users who have been given a right to do that, by the administrator. A crawler can never access your database, unless it is printed on a webpage by a script.
So when, you run a forum or a blog, the comments for that particular post are taken from the database and printed on the page by a web app programming language such PHP or ASP. Since it has been printed on the page, so your users and the crawlers can view it now.
All blogging platforms/scripts or plugins obviously print the comments so that users can view it and so will the crawlers, so you do not have do anything more. The only exception to this is Facebook Comments, as they are loaded via JS, but there are scripts to fix this too.