Question

I have a table with two fields candidate_id and keyword_id. I needs to list all the candidates/candidate_id having keyword_id 5 and 6. Somebody please help me to write a single mysql query for this please.

Answer 1

An option that is simple to adapt to more than just two keyword_id’s would be…

SELECT
  candidate_id
FROM
  yourTable
WHERE
  keyword_id IN (1,2)
GROUP BY
  candidate_id
HAVING
  COUNT(DISTINCT keyword_id) = 2

An alternative could be…

SELECT
  k1.candidate_id
FROM
  yourTable  AS k1
INNER JOIN
  yourTable  AS k2
    ON k1.candidate_id = k2.candidate_id
WHERE
      k1.keyword_id = 1
  AND k2.keyword_id = 2

It should be noted, however, that neither of these options scales very well (as you increase the number of candidates, keywords, and number of keywords that you search for.

How to deal with this poor scaling totally depends on your data, and your needs. It’s too broad and subjective a topic to discuss in a SO answer.