Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE node_table (nodekey, nodename) AS
SELECT 0, 'node0' FROM DUAL UNION ALL
SELECT 1, 'node1' FROM DUAL UNION ALL
SELECT 2, 'node2' FROM DUAL UNION ALL
SELECT 3, 'node3' FROM DUAL UNION ALL
SELECT 4, 'node4' FROM DUAL;
5 rows affected

CREATE TABLE link_table (sourcekey, sourcetarget, sourcevalue) AS
SELECT 0, 2, 2 FROM DUAL UNION ALL
SELECT 1, 2, 2 FROM DUAL UNION ALL
SELECT 1, 3, 2 FROM DUAL UNION ALL
SELECT 0, 4, 2 FROM DUAL UNION ALL
SELECT 2, 3, 2 FROM DUAL UNION ALL
SELECT 2, 4, 2 FROM DUAL UNION ALL
SELECT 3, 4, 4 FROM DUAL;
7 rows affected

SELECT json_object (
         KEY 'nodes' VALUE (
           SELECT json_arrayagg(
                    json_object(
                      KEY 'node' VALUE nodekey,
                      KEY 'name' VALUE nodename
                    )
                  )
           FROM   node_table
         ),
         KEY 'links' VALUE (
           SELECT json_arrayagg(
                    json_object(
                      KEY 'source' VALUE sourcekey,
                      KEY 'target' VALUE sourcetarget,
                      KEY 'value'  VALUE sourcevalue
                    )
                  )
           FROM   link_table
         )
       ) AS mydata
FROM   dual;


MYDATA


{"nodes":[{"node":0,"name":"node0"},{"node":1,"name":"node1"},{"node":2,"name":"node2"},{"node":3,"name":"node3"},{"node":4,"name":"node4"}],"links":[{"source":0,"target":2,"value":2},{"source":1,"target":2,"value":2},{"source":1,"target":3,"value":2},{"source":0,"target":4,"value":2},{"source":2,"target":3,"value":2},{"source":2,"target":4,"value":2},{"source":3,"target":4,"value":4}]}