in postgres select, return a column subquery as an array?

arrays postgresql subquery aggregate-functions

cc young · Jul 16, 2015 · Viewed 16.3k times · Source

(have done this before, but memory fades, as does goggle)

wish to get select from users with the tag.tag_ids for each user returned as an array.

select usr_id,
       name,
       (select t.tag_id from tags t where t.usr_id = u.usr_id) as tag_arr
from   users u;

with the idea embedded query tag_arr would be an array

Answer

Use the aggregate function:

select
    usr_id, 
    name, 
    array_agg(tag_id) as tag_arr
from users
join tags using(usr_id)
group by usr_id, name

or an array constructor from the results of a subquery:

select
    u.usr_id, 
    name, 
    array(
        select tag_id 
        from tags t 
        where t.usr_id = u.usr_id
        ) as tag_arr
from users u

The second option is simple and fast while the first one is more generic, especially convenient when you need more than one aggregate from a related table.

in postgres select, return a column subquery as an array?

Answer

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