Postgres query to get all the children ids

Question

I'm an SQL noob and wrote only very basic queries so far.

I have a table that looks like this

item_full_name      varchar(65535)
item_id             bigint
item_owners         varchar(255)
item_approver_group varchar(255)
item_state          varchar(255)
item_parent_id      bigint
item_children       varchar(65535)

Initially item_children is empty for all the rows but each item has a item_parent_id and is not null. I want to write a query that looks at all the rows & corresponding parent ids and updates each row's item_children with a string of children ids separated by comma.

for eg.

item_full_name | item_id | item_owners | item_parent_id | item_children
item1          | 1       | o1, o2      | 2              | 
item2          | 3       | owner8      | 2              |
item3          | 2       | owner6      | 0              |
item4          | 4       | owner7      | 1              |

This should be transformed to

item_full_name | item_id | item_owners | item_parent_id | item_children
item1          | 1       | o1, o2      | 2              | 4
item2          | 3       | owner8      | 2              |
item3          | 2       | owner6      | 0              | 3,1
item4          | 4       | owner7      | 1              |

Any pointers would be helpful. Thanks!

Answer 1

I started off going down the road of a recursive CTE, but then realized that you just want the children of each parent, at that single level. One approach is to aggregate the item_id by item_parent_id. Then, join your original table to this result to obtain the CSV list of children for each parent.

WITH cte AS (
    SELECT item_parent_id, STRING_AGG(item_id::text, ',') AS item_children
    FROM yourTable
    GROUP BY item_parent_id
)

SELECT
    t1.item_full_name,
    t1.item_id,
    t1.item_owners,
    t1.item_parent_id,
    t2.item_children
FROM yourTable t1
LEFT JOIN cte t2
    ON t1.item_id = t2.item_parent_id
ORDER BY
    t1.item_full_name;

Demo