Re: Extracting data from jsonb array?

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

On Mon, Dec 7, 2020 at 3:22 PM David G. Johnston <david.g.johnston@gmail.com> wrote:

On Mon, Dec 7, 2020 at 4:13 PM Steve Baldwin <steve.baldwin@gmail.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.