Oracle 18c

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CREATE TABLE t ( a,b ) AS
  SELECT 1, null FROM DUAL UNION ALL
  SELECT 2, 'bb' FROM DUAL;
2 rows affected

select json_object(
         KEY 'a' VALUE a,
         KEY 'b' VALUE b null on null
       ) c1,
       json_object(
         KEY 'a' VALUE a,
         KEY 'b' VALUE b absent on null
       ) c2
from   t;


C1
C2


{"a":1}
{"a":1}

{"a":2,"b":"bb"}
{"a":2,"b":"bb"}


select json_object(
         KEY 'a' VALUE a,
         KEY 'b' VALUE b absent on null
       ) c2,
       json_object(
         KEY 'a' VALUE a,
         KEY 'b' VALUE b null on null
       ) c1
from   t;


C2
C1


{"a":1,"b":null}
{"a":1,"b":null}

{"a":2,"b":"bb"}
{"a":2,"b":"bb"}

C2	C1
{"a":1,"b":null}	{"a":1,"b":null}
{"a":2,"b":"bb"}	{"a":2,"b":"bb"}