Re: Extracting data from jsonb array?

On Mon, Dec 7, 2020 at 4:49 PM Ken Tanzer <ken(dot)tanzer(at)gmail(dot)com> wrote:

>
>
> On Mon, Dec 7, 2020 at 3:22 PM David G. Johnston <
> david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
>
>> On Mon, Dec 7, 2020 at 4:13 PM Steve Baldwin <steve(dot)baldwin(at)gmail(dot)com>
>> wrote:
>>
>>> Try:
>>>
>>> select _message_body->'Charges'->>'Name' from ...
>>>
>>
>> Not so much..."Charges" is an array so "->>" doesn't do anything useful.
>>
>> The OP needs to use "json_array_elements" to navigate past the array and
>> get to the next layer of the json where ->>'Name' will then work.
>>
>>
> Thank you David. I had tried that function without much luck. But with
> your inspiration, I made progress and got to this:
>
> select
> _message_exchange_id,jsonb_array_elements(_message_body->'Charges')->>'Name'
> FROM message_import_court_case WHERE _message_exchange_id = 1296;
> _message_exchange_id | ?column?
> ----------------------+--------------------------------------------
> 1296 | Possession Of Burglary Tools
> 1296 | Burglary In The Second Degree (Commercial)
> (2 rows)
>
> But what I really want is one line per message, with the charges in an
> array. I can't seem to find the right syntax to make this work:
>
> => select
> _message_exchange_id,array_agg(jsonb_array_elements(_message_body->'Charges')->>'Name')
> FROM message_import_court_case WHERE _message_exchange_id = 1296;
> ERROR: column "message_import_court_case._message_exchange_id" must
> appear in the GROUP BY clause or be used in an aggregate function
> LINE 1: select _message_exchange_id,array_agg(jsonb_array_elements(_...
> ^
>
> => select
> _message_exchange_id,array_agg(jsonb_array_elements(_message_body->'Charges')->>'Name')
> FROM message_import_court_case WHERE _message_exchange_id = 1296 GROUP BY 1;
> ERROR: set-valued function called in context that cannot accept a set
>
> => select _message_exchange_id,(SELECT
> array_agg(jsonb_array_elements(_message_body->'Charges')->>'Name')) FROM
> message_import_court_case WHERE _message_exchange_id = 1296 GROUP BY 1;
> ERROR: set-valued function called in context that cannot accept a set
>
> => select _message_exchange_id,(SELECT
> array_agg(jsonb_array_elements(_message_body->'Charges')->>'Name')) FROM
> message_import_court_case WHERE _message_exchange_id = 1296;
> ERROR: column "message_import_court_case._message_exchange_id" must
> appear in the GROUP BY clause or be used in an aggregate function
> LINE 1: select _message_exchange_id,(SELECT array_agg(jsonb_array_el...
>
>
>
Sub-queries are a simple solution to get around the "set-valued function"
restriction.

The more direct way is to place the set-valued function in the FROM clause
where it wants to be, by using LATERAL (the keyword itself can be implied
when dealing with functions)

select array_agg(e->>'key') from (values
('[{"key":"val"},{"key":"val2"}]'::jsonb)) val (v), jsonb_array_elements(v)
jae (e)

David J.