Search query for node content join taxonomy term
Finally I managed to get results by using this code 
db_query("SELECT faq_questions.question, faq_questions.nid,taxonomy_index.tid
FROM faq_questions
JOIN taxonomy_index ON faq_questions.nid = taxonomy_index.nid
JOIN taxonomy_term_data ON taxonomy_index.tid =taxonomy_term_data.tid
WHERE question LIKE '%$keys%' AND taxonomy_term_data.tid='$tid'");