There is a relation-algebra tag, which is also used for questions about relational algebra, which is another thing. Operators defined in relation algebra are different than the ones defined on relational algebra. Relational algebra also deals with concepts that relation algebra does not (like functional dependences and normalization). Even wikipedia, in its article for relation algebra, it says "not to be confused with relational algebra" :P
So, what is the best thing to do about this? Is there a critical number of questions regarding a topic, as a condition for a tag about this topic to be created?
Renaming the tag (if possible) is not a good idea in my opinion, since there are also questions about relation algebra under this tag.
Edit: I just noticed there is an entry at the template of comments, for wrong tag usage. But if I'm going to correct people on their tags, I'll have to suggest them the appropriate tags. Which is something I can't do in this case, because there is no relational-algebra tag to suggest...