Snippets

Create a comma separated list of columns from a select statement

Method 1:

SELECT parent_id,
       RTRIM(XMLAGG(XMLELEMENT(e,child_id || ',')).EXTRACT('//text()'),',') AS "Children"
FROM   parentChildTable
WHERE  parent_id = 0
GROUP  BY parent_id
/

or

SELECT parent_id,
       LISTAGG(child_id, ',') WITHIN GROUP (ORDER BY child_id) AS "Children"
  FROM parentChildTable
 WHERE parent_id = 0
 GROUP BY parent_id
/

Method 2 (undocumented and cannot therefore be relied on to continue working in the same manner):

SELECT wmsys.wm_concat(<column_name>)
FROM <table_name>
/

References

stackoverflow.com