How to aggregate on a left join in a Postgres CTE?

Question

In this CTE, each row in mytable can have 0 or many rows joined to it in jointable . I'm trying to returning an array_agg of the jointable 's value column in this query, but I get an error saying I can't have an aggregate in a RETURNING .

WITH updated as(
    UPDATE mytable SET status = 'A'
FROM
  (
    SELECT id FROM mytable
    WHERE status = 'B'
    ORDER BY mycolumn
    LIMIT 100
    FOR UPDATE
  ) sub
  LEFT JOIN jointable j USING (id)
WHERE mytable.id = sub.id
GROUP BY (mytable.id)
RETURNING mytable.id, array_agg(j.value)
)
select *
from updated
ORDER BY mycolumn

Answer 1

You cannot have a GROUP BY clause in an UPDATE statement. Also, the UPDATE won't necessarily visit all matching rows in the joined table, so it wouldn't be able to return then anyway.

You will have to join jointable again in the outer query to get the desired result.