Hello Tobias,
If you want to get the tags of a particular group, you could use:
select tagname from jivetag join jivetagcloud on jivetag.tagid=jivetagcloud.tagid where jivetagcloud.containertype=700 and jivetagcloud.containerid=<id>;
This should give you the tag names that are contained in the container with the ID <id>, which you substitute with the actual ID. You can find your social group IDs in the Admin Console under People > Group Summary. Social groups have the containertype 700, so if you want to look for other types of containers, you can find them in this document: Content Types and You. Hope that helps.
Thanks,
Kevin