Should I retag a question with a tag that is based on the answer and not the question? [duplicate]

Question

Here's a scenario:

Someone asked about "How to build a SQL query to get count and blah blah blah..."

His only tag is mysql. I added the sql and count tags to it.

Someone else answered with a nice SQL statement that has a "group by" statement which seems "logically inevitable".

I proceed to add a tag group-by. I did that on the basis that someone who searches for group-by will be able to find this post useful, just as I did. My concern is, it wasn't described at all in the question, nor was it implied, unlike sql and count.

What is the community opinion?

The scenario above is a real one here. And the one who answered it nicely is Jon Skeet.

Answer 1

Why not?

There are five tags to be used on a question, so use them.

When an answer gets accepted, it becomes fused (until no longer accepted) to the question.

A relevant tag to couple the question and answer together with a concept sounds perfectly valid after the fact.

Tags also help in finding related questions and possible dupes.

Answer 2

I'd expect that if the user had realized GROUP BY clauses were relevant to his question, he wouldn't have felt the need to ask the question at all :-) But it looks like there's more to his question, according to his comments on Jon's answer, hence this kind of retagging might indeed do the OP a favor and help users follow up on his question.