PostgreSQL - transform varchar set to json - postgresql

Question

One of the columns from my table contains varchar data in the form:
{"item1", "item2", "item3"}. The number of elements may vary, the column is also nullable.
I would like to change the type of this column to json. After the change, the data should be in the format:
[{"name": "SOME_NAME", "item": "item1"}, {"name": "SOME_NAME", "item": "item2"}, {"name": "SOME_NAME", "item": "item3"}]
Value SOME_NAME is fixed and will be the same for all elements.
So having:
+-----------------+
| items (varchar) |
+-----------------+
| {item1,item2} |
| null |
| {item3,item4} |
+-----------------+
I'd like to have a new column with:
+----------------------------------------------------------------------------------+
| new_items (json) |
+----------------------------------------------------------------------------------+
| [{"name": "SOME_NAME", "item": "item1"}, {"name": "SOME_NAME", "item": "item2"}] |
| null |
| [{"name": "SOME_NAME", "item": "item3"}, {"name": "SOME_NAME", "item": "item4"}] |
+----------------------------------------------------------------------------------+
I tried with json_build_object() and similar json functions, but I didn't get the intended effect.

Answer 1

I believe you can get your data back as array of text with
with first_sel as (
select '{"item1", "item2", "item3"}' col_text
)
select col_text::text[] from first_sel;
result
col_text
---------------------
{item1,item2,item3}
Get them in different rows with the unnest
with first_sel as (
select '{"item1", "item2", "item3"}' col_text
)
select unnest(col_text::text[]) from first_sel;
Result
unnest
--------
item1
item2
item3
Then you can build the single JSON entries with json_build_object
with first_sel as (
select '{"item1", "item2", "item3"}' col_text
),
list_of_rows as(
select unnest(col_text::text[]) items from first_sel
)
select
json_build_object('SOME_NAME',items) from list_of_rows
Result
json_build_object
-------------------------
{"SOME_NAME" : "item1"}
{"SOME_NAME" : "item2"}
{"SOME_NAME" : "item3"}
(3 rows)
and then aggregate them with json_agg
with first_sel as (
select '{"item1", "item2", "item3"}' col_text
),
list_of_rows as(
select unnest(col_text::text[]) items from first_sel
),
single_entries as (
select json_build_object('SOME_NAME',items) json_obj from list_of_rows)
select
json_agg(json_obj)
from single_entries;
Result
json_agg
-----------------------------------------------------------------------------
[{"SOME_NAME" : "item1"}, {"SOME_NAME" : "item2"}, {"SOME_NAME" : "item3"}]
Edit: since the subject of the original question now spans more rows. If you have a column id for which you can group by, then use that, otherwise you can:
Create a similar table with the following
create table test (col_text text);
insert into test values ('{"item1", "item2", "item3"}');
insert into test values (NULL);
insert into test values ('{"item4", "item5", "item6"}');
Use the ctid as group by information
with list_of_rows as(
select ctid id, unnest(col_text::text[]) items from test
),
single_entries as (
select id, json_build_object('SOME_NAME',items) json_obj from list_of_rows)
select
id,
json_agg(json_obj)
from single_entries
group by id;
result
id | json_agg
-------+-----------------------------------------------------------------------------
(0,1) | [{"SOME_NAME" : "item1"}, {"SOME_NAME" : "item2"}, {"SOME_NAME" : "item3"}]
(0,3) | [{"SOME_NAME" : "item4"}, {"SOME_NAME" : "item5"}, {"SOME_NAME" : "item6"}]
(2 rows)