MySQL 8.0

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select version();


version()


8.0.30


create table currency (datetime date, rupee varchar(10), pound varchar(10));
insert into currency values ('2019-02-16', "80%", "70%"), ('2019-02-17', "30%", "60%"), ('2019-02-18', "67%", "89%");
SELECT * FROM currency;

Records: 3  Duplicates: 0  Warnings: 0




datetime
rupee
pound


2019-02-16
80%
70%

2019-02-17
30%
60%

2019-02-18
67%
89%


with cte as (
  select json_array(datetime, rupee) AS obj, rupee as value_currency, 'rupee' as currency
  from currency
  UNION ALL 
  select json_array(datetime, pound), pound, 'pound'
  from currency
),
cte2 as (
  select currency, 
        concat('[',
          group_concat(
            obj
            order by value_currency
          ),
          ']'
        ) obj
  from cte
  group by currency
)
select json_arrayagg(obj)
from (
  select json_object(
                  'currency', currency, 
                  'data', CAST( obj as JSON)
       ) AS obj
  from cte2
) x


json_arrayagg(obj)


[{"data": [["2019-02-17", "60%"], ["2019-02-16", "70%"], ["2019-02-18", "89%"]], "currency": "pound"}, {"data": [["2019-02-17", "30%"], ["2019-02-18", "67%"], ["2019-02-16", "80%"]], "currency": "rupee"}]