Answered: what mysql query should I use to fetch the tag of particular question id ?

Use a query similar to the one I suggested here, but instead of filtering for a tag-name and selecting the post ID do it the other way around:

SELECT w.word
FROM qa_posts q
  INNER JOIN qa_posttags t ON q.postid=t.postid
  INNER JOIN qa_words w ON t.wordid=w.wordid
WHERE q.postid=42;