Is there a way (e.g. with SEDE) to find questions with an existing tag that's been synonymized (but not merged) into another tag?

Question

Say Tag 1 has been added as a synonym of Tag 2, so that new questions attempting to use Tag 1 will be automatically pointed to Tag 2. However, the two tags haven't been merged (yet). Then, if you click on Tag 1 or attempt to search for it, you'll automatically be redirected to the tag page for Tag 2.

As stated in the FAQ on tag synonyms and merged tags:

All good tag synonyms should eventually be merged (source). This retags all non-deleted questions tagged with another tag – which is not automatically done when a synonym is created. As merging is not (easily) reversible, it should be done with caution. Only moderators can perform a merge.

However, it can sometimes be tricky to evaluate whether a pair of tags should be merged. Part of that process usually involves seeing how the tags are currently being used, e.g. if they're being used to refer to the same thing. If both tags are only on a few questions each, you can do this pretty easily by just scrolling through the page for Tag 2. However, if Tag 1 is on a lot of questions and Tag 2 is only on a few questions (or vice versa), this isn't quite as feasible – you might have to go through dozens of pages to even find a single question with the less popular tag.

There doesn't seem to be an obvious way on sites to find questions listed separately for the main tag vs. its synonyms. As such, I was wondering if it's possible to use SEDE to find this information.

How can I get a list of questions with an existing tag that's been synonymized (but not merged) into another tag? (SEDE seems like a possible way to get this information, but I'm open to other solutions as well.)

Answer 1

As far as I can tell, SEDE indeed can help with this - all you need is to find the questions with the specific tag. If the question still has the original tag, from SEDE you get the question with this tag- not with the master tag. (In the other words, SEDE keeps the same tags as displayed on questions - so if the tags weren't merged, the synonym and the master tag still retain their questions.)

So all you need is:

SELECT q.Id as [Post Link], 
  q.Tags, q.CreationDate, q.LastActivityDate
FROM Posts q INNER JOIN PostTags pt ON q.Id=pt.PostId
INNER JOIN Tags t ON t.id = pt.TagId
WHERE t.TagName=##tagName##
ORDER BY q.CreationDate DESC

Naturally, you can add further information about the questions and change the ordering in whichever way you please.

Using the PostTags table should be faster than just comparing q.Tags LIKE '%<##Tagname##>%'. (Although it is worth pointing out that the table PostTags doesn't contain information about deleted questions - so if you are interested in those, you can't rely on this table. At least this is how it works on publicly available instance of SEDE - as mentioned in the comments, the devs have access to more data about deleted questions.)

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Let us take careers-germany as an example. It is a synonym with the master tag legacy-stack-overflow-jobs. At the moment, I get from SEDE 7 questions tagged careers-germany and 2172 questions tagged legacy-stack-overflow-jobs. (Probably it is worth checking after the next update of SEDE - there is a small difference between the numbers from SEDE and the numbers I see on the site.)

The 7 questions tagged careers-germany, have the tag legacy-stack-overflow-jobs, too. So the query that looks for questions with the synonym but not with the master tag returns no questions in this case.

Interestingly, when I go to see how many questions tagged legacy-stack-overflow-jobs are on the site, I get 2181 questions when viewing the active tab and 2172 questions when viewing the newest tab.

Similarly, searching for [legacy-stack-overflow-jobs] is:q returns only 2172 questions.

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Maybe some smaller example would be easier to work with - let us try canonical-questions and canonical. Using the same query, I get 12 questions tagged canonical-questions and 22 questions tagged canonical. In this case, there is a single question with both tags and 11 question tagged canonical-questions but not canonical.

Searching for [canonical] is:q returns 22 questions. (This is consistent with the behavior described in Makyen's comment - search only returns the question with the master tag, not with the synonym. Perhaps this could be considered a bug - as previously reported here.)

The tag canonical shows 34 questions on the active tab and 22 questions on the newest tab.

Actually, I think that this is a manifestation of another bug - the page with the active tab says "34 questions" but this is small enough to count manually - I only see 33. (So the one question with both tags is probably counted twice. Here is a small SEDE query showing the questions with one of these tags ordered in the same way as in the active tab.)