How can multiple rows be concatenated into one in

Question

This question already has an answer here:

• SQL Query to concatenate column values from multiple rows in Oracle 11 answers

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

Easy:

SELECT question_id, wm_concat(element_id) as elements
FROM   questions
GROUP BY question_id;

Pesonally tested on 10g ;-)

From http://www.oracle-base.com/articles/10g/StringAggregationTechniques.php

Answer 2

There are many way to do the string aggregation, but the easiest is a user defined function. Try this for a way that does not require a function. As a note, there is no simple way without the function.

This is the shortest route without a custom function: (it uses the ROW_NUMBER() and SYS_CONNECT_BY_PATH functions )

SELECT questionid,
       LTRIM(MAX(SYS_CONNECT_BY_PATH(elementid,','))
       KEEP (DENSE_RANK LAST ORDER BY curr),',') AS elements
FROM   (SELECT questionid,
               elementid,
               ROW_NUMBER() OVER (PARTITION BY questionid ORDER BY elementid) AS curr,
               ROW_NUMBER() OVER (PARTITION BY questionid ORDER BY elementid) -1 AS prev
        FROM   emp)
GROUP BY questionid
CONNECT BY prev = PRIOR curr AND questionid = PRIOR questionid
START WITH curr = 1;

Answer 3

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;

Answer 4

This OTN-thread contains several ways to do string aggregation, including a performance comparison: http://forums.oracle.com/forums/message.jspa?messageID=1819487#1819487