Re: array_to_json/array_agg question

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