From this post, I understand that there is no real distinction between a Schengen tourist visa and a Schengen business visa as long as they are both for short stay only (Type C). So, if I have an existing tourist visa can I also attend a business trip (training/meeting/conference)?
There is no sharp distinction between tourism and business visas in the Schengen area. So if you have a multiple entry visa with long validity it's perfectly fine to use for both purposes and more (business or tourism are only two of half a dozen purposes like medical travel, cultural travel or short studies).
On the other hand, a single or double entry visa is issued for a specific trip. Even if the details are not mentioned on the visa sticker, you are not expected to make any major change to this trip, like going to another country or changing its purpose. In practice, YMMV of course.
See also Use of unused Schengen visa to travel to Switzerland for someone's first-hand experience with a similar issue.