Oracle JSON_OBJECT NULL ON NULL clause not working

Question

I'm trying to get Oracle to produce JSON null values on SQL NULL data, as follows:

select 
  json_object(key 'a' value 1, key 'b' value null null on null)   c1, 
  json_object(key 'a' value 1, key 'b' value null absent on null) c2
from dual;

Or also:

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 (
  select 1 a, null b
  from dual
) t;

Unfortunately, both queries result in:

|C1        |C2        |
|----------|----------|
|{"a":1}   |{"a":1}   |

I would have expected this, instead:

|C1                 |C2        |
|-------------------|----------|
|{"a":1,"b":null}   |{"a":1}   |

What am I missing? I'm using Oracle XE 18c

Answer 1

There appears to be a bug on how Oracle handles JSON_OBJECT and it will take the parameter from the last JSON_on_null_clause in the statement and apply it to all of the JSON_OBJECT expressions:

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

If you do this:

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;

Then the output is:

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

If you perform the same query with the expressions reversed:

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;

Then the output is:

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

db<>fiddle here

Answer 2

you found a bug. I have filed Bug 31013529 - TWO JSON_OBJECT WITH DIFFERENT ON NULL HANDLER RETURN WRONG RESULTS

We'll fix it asap and include it in the bundle patches. Let us know if you need a one off patch

thanks b