Thread: array_to_json/array_agg question

Hi

Before I go down the road of taking care of this in the front-end through iterations, I thought I would ask the pgsql
ifthere was a clever query I could do on postgres that would take care of it for me instead. 

In essence, I would like to consolidate values from the same key as a json array, so instead of :
[{"key":"one","value_1":"foo","value_2":"foo"},{"key":"one","value_1":"foo","value_2":"bar"}]
I would have (forgive me if my JSON syntax is incorrect here) :
[{"key":"one",[{"value_1":"foo","value_2":"foo"},{"value_1":"foo","value_2":"bar"}]}]


A simplified example of where I am at the moment:


create table test_a(key text,value_1 text,value_2 text);insert into test_a(key,value_1,value_2)
values('one','foo','foo');
insert into test_a(key,value_1,value_2) values('one','foo','bar');
insert into test_a(key,value_1,value_2) values('two','bar','foo');
select array_to_json(array_agg(row_to_json(p))) from (select * from test_a where key='one') p;
[{"key":"one","value_1":"foo","value_2":"foo"},{"key":"one","value_1":"foo","value_2":"bar"}]

On 2024-02-20 10:48 +0100, Laura Smith wrote:
> Before I go down the road of taking care of this in the front-end
> through iterations, I thought I would ask the pgsql if there was a
> clever query I could do on postgres that would take care of it for me
> instead.
> 
> In essence, I would like to consolidate values from the same key as a json array, so instead of :
> [{"key":"one","value_1":"foo","value_2":"foo"},{"key":"one","value_1":"foo","value_2":"bar"}]
> I would have (forgive me if my JSON syntax is incorrect here) :
> [{"key":"one",[{"value_1":"foo","value_2":"foo"},{"value_1":"foo","value_2":"bar"}]}]
> 
> A simplified example of where I am at the moment:
> 
> create table test_a(key text,value_1 text,value_2 text);insert into test_a(key,value_1,value_2)
values('one','foo','foo');
> insert into test_a(key,value_1,value_2) values('one','foo','bar');
> insert into test_a(key,value_1,value_2) values('two','bar','foo');
> select array_to_json(array_agg(row_to_json(p))) from (select * from test_a where key='one') p;
> [{"key":"one","value_1":"foo","value_2":"foo"},{"key":"one","value_1":"foo","value_2":"bar"}]

You almost got the subrecord ("value_1" and "value_2") right.  You need
to use json_build_object() (or even the new json_object() function added
in pg16) instead of row_to_json() to just include "value_1" and
"value_2".  Then GROUP BY "key" and aggregate the subrecords with
json_agg().  Then build the top-level record ("key" and "values") with
json_build_object().  And finally one more aggregation with json_agg()
to get a single array.

-- 
Erik

> You almost got the subrecord ("value_1" and "value_2") right. You need
> to use json_build_object() (or even the new json_object() function added
> in pg16) instead of row_to_json() to just include "value_1" and
> "value_2". Then GROUP BY "key" and aggregate the subrecords with
> json_agg(). Then build the top-level record ("key" and "values") with
> json_build_object(). And finally one more aggregation with json_agg()
> to get a single array.
>


Interesting ideas, thanks Erik.

Subsequent to my original posting, but prior to your reply and based on an off-list idea from someone else, I came up
withthe following adaptation: 

    SELECT json_agg(q) INTO v_res FROM (SELECT array_to_json(array_agg(row_to_json(p))) AS q
    FROM (SELECT * FROM test_a)p group by key)s;

But maybe I should be considering json_build_object() instead, or maybe json_object() (although I'm currently on 14.5,
soit would require an upgrade to 16 first, which is possible as a longer-term option, but right now I'm developing
againsta 14.5 backend). 

On 2024-02-20 10:48 +0100, Laura Smith wrote:
> Before I go down the road of taking care of this in the front-end
> through iterations, I thought I would ask the pgsql if there was a
> clever query I could do on postgres that would take care of it for me
> instead.
>
> In essence, I would like to consolidate values from the same key as a json array, so instead of :
> [{"key":"one","value_1":"foo","value_2":"foo"},{"key":"one","value_1":
> "foo","value_2":"bar"}] I would have (forgive me if my JSON syntax is
> incorrect here) :
> [{"key":"one",[{"value_1":"foo","value_2":"foo"},{"value_1":"foo","val
> ue_2":"bar"}]}]
>
> A simplified example of where I am at the moment:
>
> create table test_a(key text,value_1 text,value_2 text);insert into
> test_a(key,value_1,value_2) values('one','foo','foo'); insert into
> test_a(key,value_1,value_2) values('one','foo','bar'); insert into
> test_a(key,value_1,value_2) values('two','bar','foo'); select
> array_to_json(array_agg(row_to_json(p))) from (select * from test_a
> where key='one') p;
> [{"key":"one","value_1":"foo","value_2":"foo"},{"key":"one","value_1":
> "foo","value_2":"bar"}]

May be something like this
SELECT jsonb_agg( value )
FROM ( SELECT jsonb_build_object( 'key', key, 'value', jsonb_agg( jsonb_build_object( 'value_1', value_1, 'value_2',
value_2) ) ) FROM test_a GROUP BY key ) per_key( value )