How can multiple rows be concatenated into one in Oracle without creating a stored procedure?

Question

How can I achieve the following in oracle without creating a stored procedure?

Data Set:

question_id    element_id
1              7
1              8
2              9
3              10
3              11
3              12

Desired Result:

question_id    element_id
1              7,8
2              9
3              10,11,12

Answer 1

From Oracle 11gR2, the LISTAGG clause should do the trick:

SELECT question_id,
       LISTAGG(element_id, ',') WITHIN GROUP (ORDER BY element_id)
FROM YOUR_TABLE
GROUP BY question_id;

Beware if the resulting string is too big (more than 4000 chars for a VARCHAR2, for instance): from version 12cR2, we can use ON OVERFLOW TRUNCATE/ERROR to deal with this issue.