How to process json data and put it into arrays in postgresql - sql

Question

I have a postgresql table with two columns. name VARCHAR(255) and notes JSON. A sample dataset might look like this:
| name | notes |
|-----------|----------------------------------|
| 'anna' | {'link_to': ['bob']} |
| 'bob' | {'link_to': ['anna', 'claudia']} |
| 'claudia' | {'link_to': []} |
Now I want to do two things.
put the list from the json at 'link_to' into another column called referrals_to (which must be of array VARCHAR type then). From my example:
name
notes
referrals_to
'anna'
{'link_to': ['bob']}
['bob']
'bob'
{'link_to': ['anna', 'claudia']}
['anna', 'claudia']
'claudia'
{'link_to': []}
[]
create another column called referrals_from, where I want to store all names from which a name was referred. In my example:
name
notes
referrals_from
'anna'
{'link_to': ['bob']}
['bob']
'bob'
{'link_to': ['anna', 'claudia']}
['anna']
'claudia'
{'link_to': []}
['bob']
How do I do this using postgresql queries? I could easily do it using python, but this would be slower than using postgresql directly, I guess.

Answer 1

The referrals_to column can be done using the -> operator to extract the array. For the referrals_from column I would use a scalar sub-select to collect all referring names into a single column (a JSON array)
select name,
notes,
notes -> 'link_to' as referrals_to,
(select jsonb_agg(name)
from the_table t2
where t2.name <> t1.name
and t2.notes -> 'link_to' ? t1.name) as referrals_from
from the_table t1
;
The ? operator tests if a JSON array contains a specific string. In this case it tests if the link_to array contains the name from the outer query.
Online Example
Assuming name is unique in your table you can use that query to update the new columns in the table:
update the_table
set referrals_to = notes -> 'link_to',
referrals_from = t.referrals_from
from (
select t1.name,
(select jsonb_agg(name)
from the_table t2
where t2.name <> t1.name
and t2.notes -> 'link_to' ? t1.name) as referrals_from
from the_table t1
) t
where t.name = the_table.name;

Answer 2

More or less what you described in your steps:
WItH cte AS (
SELECT
name,
BTRIM(value:: text, '"') AS referrals
FROM
(
SELECT
name,
foo.notes:: json - > > 'link_to' AS link_to
FROM
foo
) a
left JOIN LATERAL json_array_elements (a.link_to:: json) ON TRUE
)
SELECT
a1.name,
array_to_json(array_agg(a1.referrals))
FROM
cte a1
JOIN cte a2 ON a1.name = a2.referrals
GROUP BY
a1.name