Get cutted JSON in query - postgresql

Question

I have a column in db which constains JSON values like:
{"key-1": "val-1", "key-2": "val-2", "key-3": "val-3"}
By query like..
SELECT column->>'key-1' FROM table;
I can get my val-1.
Is there a way to get value with key as JSON in sql query from already existed JSON value?
I want to get result like:
{"key-1": "val-1"}
from
{"key-1": "val-1", "key-2": "val-2", "key-3": "val-3"}
using sql query.

Answer 1

Use ampersand operator, &, e.g.,
Live test: https://www.db-fiddle.com/f/9izCEH75JhwVDvsGvsZomG/0
with the_table as
(
select '{"key-1": "val-1", "key-2": "val-2", "key-3": "val-3"}'::jsonb as d
)
select d & 'key-1' as j from the_table
Output:
| j |
| ----------------- |
| {"key-1":"val-1"} |
Just kidding :) Create a function that extracts the desired key value pair, and then create your own user-defined operator for it.
create or replace function extract_one_jsonb(j jsonb, key text)
returns jsonb
as
$$
select jsonb_build_object(key, j->key)
$$ language sql;
create operator & (
leftarg = jsonb,
rightarg = text,
procedure = extract_one_jsonb
);
Of course you can just use a function, or if creating a user-defined operator is not an option:
with the_table as
(
select '{"key-1": "val-1", "key-2": "val-2", "key-3": "val-3"}'::jsonb as d
)
select extract_one_jsonb(d, 'key-1') as j from the_table
Output:
| j |
| ----------------- |
| {"key-1":"val-1"} |
If extracting a key value pair from jsonb is being done many times, it's desirable to give an operator for it, e.g., &. Postgres is pretty flexible when you want to create your own operator, this can be created too: ->>>.
Live test: https://www.db-fiddle.com/f/9izCEH75JhwVDvsGvsZomG/1
create operator ->>> (
leftarg = jsonb,
rightarg = text,
procedure = extract_one_jsonb
);
Output:
| j |
| ----------------- |
| {"key-1":"val-1"} |
->> is already used by Postgres: https://www.postgresql.org/docs/11/functions-json.html
You can create '->>>' instead. ->>> looks more like an extractor operator than ampersand &. Besides it looks good even you stick it to the source field (that is without spaces)
with the_table as
(
select '{"key-1": "val-1", "key-2": "val-2", "key-3": "val-3"}'::jsonb as d
)
select d->>>'key-1' as j from the_table
Tried the following, it works too, looks like a scissor (for cutting): %>
select d%>'key-1' as j from the_table

Answer 2

The only thing I can think of is to get the key/value pair and assemble that back into a single JSON value:
select jsonb_build_object(j.k, j.v)
from the_table t, jsonb_each(t.json_col) as j(k,v)
where j.k = 'key-1'
and ... more conditions ...;
Online example: https://rextester.com/VGSX43955