Question

How can you select question_id and tags for a question when one of the tags is known?

I am building a tag-search which searches questions by one tag such that the result shows the question and its tags including the one which was used in the search.

I use the same tables as in this question.
They are

Tables

questions          |     tags
-------------------|-----------------
  question_id      |     tag
  title            |     question_id
  was_sent_at_time |

The query

    SELECT question_id, tag
    FROM tags
    WHERE question_id IN 
    ( 
        SELECT question_id
        FROM questions
        ORDER BY was_sent_at_time
        DESC LIMIT 50
    )
    AND tag = $1;          // Problem here   

The problem with this query is that it does not show other tags assigned to the question.
It may be possible to get the question_id and tags if there exists a given tag.

Answer 1

    SELECT TOP 50 q.question_id, q.title, t.tag
    FROM tags t
    INNER JOIN questions q
    ON t.question_id = q.question_id
    WHERE q.question_id IN 
    ( 
        SELECT tin.question_id
        FROM tags tin
        WHERE tin.tag = $1
    )
   ORDER BY q.was_sent_at_time DESC

That answers your question (if I understood it correctly), but I think you will have too much duplicate data – you don’t really want the question title repeated for each tag. So you should break it out into 2 result sets:

    SELECT TOP 50 q.question_id, q.title
    FROM questions q
    WHERE q.question_id IN 
    ( 
        SELECT tin.question_id
        FROM tags tin
        WHERE tin.tag = $1
    )
   ORDER BY q.was_sent_at_time DESC

and:

  SELECT t.question_id, t.tag
    FROM tags t
    WHERE t.question_id IN 
    ( 
        /* 
       SAME SQL AS ABOVE WITH SELECT q.question_id, 
       OR SELECT FROM A TEMP TABLE THAT ABOVE WAS SAVED TO
       */
    )
   ORDER BY t.question_id

Then when you build your page or whatever, you would bring them together.