There is a 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...

  • $\begingroup$ When I look at the questions tagged (relation-algebra) at this moment, it seems that there are not too many of them which are really about relation algebras as a special kind of Boolean algebras. I might have missed some of them, but I only see these three questions: 1, 2, 3. $\endgroup$ Feb 28, 2014 at 7:07
  • $\begingroup$ Most of the questions in this tag could either be understood as questions about relational algebra, or the tag could be omitted and they could be left with (relations) tag. $\endgroup$ Feb 28, 2014 at 7:11
  • $\begingroup$ A new related post in 2021 tag management thread. $\endgroup$ Mar 6, 2021 at 6:48

2 Answers 2


My suggestion to deal with this tag would be following:

  • Rename to . (This has to be done by a moderator.)
  • Retag the few questions that are actually or . (I think that plurals are preferred for tags. Maybe we could have both of them as synonyms)1
  • Retag the few questions which are neither about relation algebras nor about relational algebra in a such way that neither of this tags remains there. (The tag can be added, if the question does not already have this tag.)

After creating the two tags, it is also useful to:

  • Create tag-excerpts and tag-wikis. It would be good to mention in the tag-excerpt the other tag, too. (In a similar way that some other tags have information about tags with similar name or tags which are commonly mistakenly applied. For example, tag-excerpt for graph-theory says: Use (graphing-functions) instead if your question is about graphing or plotting functions.)
  • Create a comment template which can be used to explain proper choice of tag.

1 I am not sure whether a separate tag for s is needed. However, since it has been already created and it has been used in a few questions in the correct way, I think that it could be kept. Probably it would be good to use it along with the tag .


Maybe making the decision about what to do is going to be easier if we check, how many questions tagged are not about relational algebra. I have made this answer CW - feel free to other questions or correct my post, if I was wrong in categorizing some of the posts.

I will include some short excerpts from Wikipedia, so that we are clear about meaning of these terms.

This is what Wikipedia says about relational algebra:

In computer science, relational algebra is an offshoot of first-order logic and of algebra of sets concerned with operations over finitary relations, usually made more convenient to work with by identifying the components of a tuple by a name (called attribute) rather than by a numeric column index, which is called a relation in database terminology.
The main application of relational algebra is providing a theoretical foundation for relational databases, particularly query languages for such databases, chief among which is SQL.

Here is beginning of the article about relation algebra:

In mathematics and abstract algebra, a relation algebra is a residuated Boolean algebra expanded with an involution called converse, a unary operation. The motivating example of a relation algebra is the algebra $2^{X^2}$ of all binary relations on a set $X$, that is, subsets of the cartesian square $X^2$, with $R•S$ interpreted as the usual composition of binary relations $R$ and $S$, and with the converse of $R$ interpreted as the inverse relation.

I can imagine that someone can use the phrase relation algebra to mean something like "operations with relations", but let us stick with the meaning given in the Wikipedia article for the purpose of this post.

Questions tagged which are about relation algebra

Questions tagged which are neither about relation algebra nor about relational algebra

  • $\begingroup$ I should perhaps add, that there are many questions where this tag is probably not needed, or which are in a gray area (i.e., I am not sure whether they should have be tagged as questions about relational area). I have tried to list at least questions where it is obvious that neither of the two possible meanings of the tag would be appropriate. $\endgroup$ Feb 28, 2014 at 10:46

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