create table notajson(n number);

select t.n, (select count(*) from json_table('[]', '$[*]' columns (x varchar2(10 char))) jt) const_expr
from notajson t
for update of t.n;

ORA-01786: FOR UPDATE of this query expression is not allowed

> select t1s.nextval, json_object(t.dummy) j from dual t;

   NEXTVAL J                             
---------- ------------------------------
       123 {"dummy":"X"}                 

> select 123, json_object(t.*) j from dual t;

       123 J                             
---------- ------------------------------
       123 {"DUMMY":"X"}                 

> select t1s.nextval, json_object(t.*) j from dual t;

ORA-02287: sequence number not allowed here

select json_object(t.DummY) j from dual t;

J                             
------------------------------
{"DummY":"X"}

Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.5.0.0.0

SQL> set serveroutput on
SQL> declare
  2    j json_element_t;
  3    c clob;
  4  begin
  5    select full_text into c from all_statements@remote where rownum=1 and length(full_text) > 0;
  6    dbms_output.put_line(substr('c='||c, 1,22));
  7    j := json_element_t.parse(c);
  8  end;
  9  /
c=SELECT COUNT(*) FROM
declare
*
ERROR at line 1:
ORA-02019: connection description for remote database not found
ORA-06512: at "SYS.JDOM_T", line 9
ORA-06512: at "SYS.JSON_ELEMENT_T", line 22
ORA-06512: at line 7

declare
  js varchar2(100 char) := '{"x":null}';
  j json_object_t;
  s varchar2(100 char);
  c clob;
begin
  select json_value(js, '$.x' returning varchar2(100 char)),
         json_value(js, '$.x' returning clob)
    into s, c
    from dual;
  dbms_output.put_line('___sql s=['||s||'] c=['||c||']');
  j := json_object_t(js);
  s := j.get_string('x'); c := j.get_clob('x');
  dbms_output.put_line('pl/sql s=['||s||'] c=['||c||']');  
end;
/

___sql s=[] c=[]
pl/sql s=[] c=[null]