Question

I have a table named “person”. It contains person’s id and it’s parent id (only one parent is possible). As a result of a query, I want a table with first column – a person id, and a second column – a list of it’s children id’s. How exactly to do this? I’ve read about listagg function, but I’m not sure if it is appropriate for my purpose.
And this query produces an empty second column:

select t1.id,
(select t2.id from person t2
where t2.parent_id = t1.id) from person t1
where t1.status = ‘parent’;

Answer 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