How to convert two rows into key-value json object in postgresql? - sql

Question

I have a data model like the following which is simplified to show you only this problem (SQL Fiddle Link at the bottom):
A person is represented in the database as a meta table row with a name and with multiple attributes which are stored in the data table as key-value pair (key and value are in separate columns).
Expected Result
Now I would like to retrieve all users with all their attributes. The attributes should be returned as json object in a separate column. For example:
name, data
Florian, { "age":23, "color":"blue" }
Markus, { "age":24, "color":"green" }
My Approach
Now my problem is, that I couldn't find a way to create a key-value pair in postgres. I tried following:
SELECT
name,
array_to_json(array_agg(row(d.key, d.value))) AS data
FROM meta AS m
JOIN (
SELECT d.fk_id, d.key, d.value AS value FROM data AS d
) AS d
ON d.fk_id = m.id
GROUP BY m.name;
But it returns this as data column:
[{"f1":"age","f2":24},{"f1":"color","f2":"blue"}]
Other Solutions
I know there is the function crosstab which enables me to turn the data table into a key as column and value as row table. But this is not dynamic. And I don't know how many attributes a person has in the data table. So this is not an option.
I could also create a json like string with the two row values and aggregate them. But maybe there is a nicer solution.
And no, it is not possible to change the data-model because the real data model is already in use of multiple parties.
SQLFiddle
Check and test out the fiddle i've created for this question:
http://sqlfiddle.com/#!15/bd579/14

Answer 1

Use the aggregate function json_object_agg(key, value):
select
name,
json_object_agg(key, value) as data
from data
join meta on fk_id = id
group by 1;
Db<>Fiddle.
The function was introduced in Postgres 9.4.

Answer 2

I found a way to return crosstab data with dynamic columns. Maybe rewriting this will be better to suit your needs:
CREATE OR REPLACE FUNCTION report.usp_pivot_query_amount_generate(
i_group_id INT[],
i_start_date TIMESTAMPTZ,
i_end_date TIMESTAMPTZ,
i_interval INT
) RETURNS TABLE (
tab TEXT
) AS $ab$
DECLARE
_key_id TEXT;
_text_op TEXT = '';
_ret TEXT;
BEGIN
-- SELECT DISTNICT for query results
FOR _key_id IN
SELECT DISTINCT at_name
FROM report.company_data_date cd
JOIN report.company_data_amount cda ON cd.id = cda.company_data_date_id
JOIN report.amount_types at ON cda.amount_type_id = at.id
WHERE date_start BETWEEN i_start_date AND i_end_date
AND group_id = ANY (i_group_id)
AND interval_type_id = i_interval
LOOP
-- build function_call with datatype of column
IF char_length(_text_op) > 1 THEN
_text_op := _text_op || ', ' || _key_id || ' NUMERIC(20,2)';
ELSE
_text_op := _text_op || _key_id || ' NUMERIC(20,2)';
END IF;
END LOOP;
-- build query with parameter filters
_ret = '
SELECT * FROM crosstab(''SELECT date_start, at.at_name, cda.amount ct
FROM report.company_data_date cd
JOIN report.company_data_amount cda ON cd.id = cda.company_data_date_id
JOIN report.amount_types at ON cda.amount_type_id = at.id
WHERE date_start between $$' || i_start_date::TEXT || '$$ AND $$' || i_end_date::TEXT || '$$
AND interval_type_id = ' || i_interval::TEXT || '
AND group_id = ANY (ARRAY[' || array_to_string(i_group_id, ',') || '])
ORDER BY date_start'')
AS ct (date_start timestamptz, ' || _text_op || ')';
RETURN QUERY
SELECT _ret;
END;
$ab$ LANGUAGE 'plpgsql';
Call the function to get the string, then execute the string. I think I tried executing it in the function, but it didn't work well.

Answer 3

I ran into the same problem when I needed to update some JSON and remove a few elements in my database. This query below worked well enough for me, as it preserves the string quotes but does not add them to numbers.
select
'{' || substr(x.arr, 3, length(x.arr) - 4) || '}'
from
(
select
replace(replace(cast(array_agg(xx) as varchar), '\"', '"'), '","', ', ') as arr
from
(
select
elem.key,
elem.value,
'"' || elem.key || '":' || elem.value as xx
from
quote q
cross join
json_each(q.detail::json -> 'bQuoteDetail'-> 'quoteHC'->0) as elem
where
elem.key != 'allRiskItems'
) f
) x