Hiding whole objects when generating JSON form ORACLE tables - sql

Question

I would like to know is there any "elegant" way to hide whole generated object when one of columns is null?
For example if "tab_2.col_c" is null (because join doesn't return any values for "tab_2"), whole "tab_2" object is hidden, but when "tab_2.col_c" contains data then object "tab_2" is displayed.
tab_1:
|id|col_a|col_b|
|--|-----|-----|
|1 | a1 | b1 |
|2 | a2 | b2 |
tab_2:
|id|fk_id|col_c|col_d|
|--|-----|-----|-----|
|11| 2 | c1 | d1 |
|22| 2 | c2 | d2 |
Query:
select
json_object(
'tab_1_a' VALUE tab_1.col_a,
'tab_2_b' VALUE tab_1.col_b,
'tab_2' value json_object('c' value tab2.col_c,
'd' value tab2.col_d
)
)
from tab_1, tab2
where tab_1.id = tab_2.fk_id(+)
group by tab_1.col_a, tab_2.col_b
Output for "tab_1.id" = 1
{
"tab_1_a": "a1",
"tab_1_b": "b1",
"tab_2": {
"c": null,
"d": null
}
}
Desired output:
{
"tab_1_a": "a1",
"tab_1_b": "b1"
}

Answer 1

If you do not want the keys to be shown, you can use the ABSENT ON NULL clause of JSON_OBJECT_T. This will cause {} to still be returned though even if all of the keys do not contain a value.
WITH
tab_1 (id, col_a, col_b)
AS
(SELECT 1, 'a1', 'b1' FROM DUAL
UNION ALL
SELECT 2, 'a2', 'b2' FROM DUAL),
tab_2 (id,
fk_id,
col_c,
col_d)
AS
(SELECT 11, 2, 'c1', 'd1' FROM DUAL
UNION ALL
SELECT 22, 2, 'c2', 'd2' FROM DUAL)
SELECT tab_1.id,
json_object (
'tab_1_a' VALUE tab_1.col_a,
'tab_2_b' VALUE tab_1.col_b,
'tab_2' VALUE
json_object ('c' VALUE tab_2.col_c,
'd' VALUE tab_2.col_d
ABSENT ON NULL)) AS formatted_json
FROM tab_1 LEFT JOIN tab_2 ON tab_1.id = tab_2.fk_id
WHERE tab_1.id = 1;
ID FORMATTED_JSON
_____ _____________________________________________
1 {"tab_1_a":"a1","tab_2_b":"b1","tab_2":{}}
If you do not want {} returned, you can use a subquery. If no results are found then it will return null, but you can hide that as well using the ABSENT ON NULL clause again.
WITH
tab_1 (id, col_a, col_b)
AS
(SELECT 1, 'a1', 'b1' FROM DUAL
UNION ALL
SELECT 2, 'a2', 'b2' FROM DUAL),
tab_2 (id,
fk_id,
col_c,
col_d)
AS
(SELECT 11, 2, 'c1', 'd1' FROM DUAL
UNION ALL
SELECT 22, 2, 'c2', 'd2' FROM DUAL)
SELECT tab_1.id,
json_object (
'tab_1_a' VALUE tab_1.col_a,
'tab_2_b' VALUE tab_1.col_b,
'tab_2' VALUE
(SELECT json_object ('c' VALUE tab_2.col_c,
'd' VALUE tab_2.col_d
ABSENT ON NULL)
FROM tab_2
WHERE tab_2.fk_id = tab_1.id)
ABSENT ON NULL) AS formatted_json
FROM tab_1
WHERE tab_1.id = 1;
ID FORMATTED_JSON
_____ __________________________________
1 {"tab_1_a":"a1","tab_2_b":"b1"}