Constraint check if row and other row not null on same time - oracle

Question

I have a school 'project' to work on, which has some tables and one table needs to have a constraint which is not working out for me.
There are some tables like QUESTION, ANSWER and REACTION.
A reaction belongs with or a question or a answer but not both on the same time.
There by I have 2 rows:
question_id NUMBER,
answer_id NUMBER,
Both not null because the cant by null, but not on the same time.
I already made a constraint but isn't working..
/* CHECK if reaction belongs to an question or a answer NOT WORKING YET*/
CONSTRAINT CHECK_question_or_answer CHECK((answer_id != NULL AND question_id = NULL) OR (answer_id = NULL OR question_id != NULL))
Already tested the constraint and I can insert a value without a question_id or answer_id.
I hope it's a bit clear, if not, I am happy yo try explain myself better.
(still newby on SQL)
Thanks.

Answer 1

Your constraint:
CONSTRAINT CHECK_question_or_answer CHECK((answer_id != NULL AND profile_id = NULL) OR (answer_id = NULL OR profile_id != NULL))
Is always FALSE.
You need to use IS NULL or IS NOT NULL like:
CONSTRAINT CHECK_question_or_answer CHECK((answer_id IS NOT NULL AND profile_id IS NULL) OR (answer_id IS NULL OR profile_id IS NOT NULL))
This is because comparison operators != , = , > , <, combined with NULL produce NULL and are treated as false.
Demo:
SELECT 1
FROM dual
WHERE 1 IS NOT NULL;
SELECT 1
FROM dual
WHERE 1 != NULL;
From doc:
NULL values represent missing or unknown data. NULL values are used as
placeholders or as the default entry in columns to indicate that no
actual data is present. The NULL is untyped in SQL, meaning that it is
not an integer, a character, or any other specific data type.
Note that NULL is not the same as an empty data string or the
numerical value '0'. While NULL indicates the absence of a value, the
empty string and numerical zero both represent actual values.
While a NULL value can be assigned, it can not be equated with
anything, including itself.
Because NULL does not represent or equate to a data type, you cannot
test for NULL values with any comparison operators, such as =, <, or
<>.
The IS NULL and IS NOT NULL operators are used to test for NULL
values.

Answer 2

Do it the other way around. Put the id of the main table in the others like that
question table
--------------
id
text
...
answers table
-------------
id
question_id
text
...
reactions table
---------------
id
question_id
text
...
And question_id is never null. Then you can use a left join to get the results from both tables - one of them will have no results.
select *
from questions q
left join answers a on a.question_id = q.id
left join reactions r on r.question_id = q.id

Answer 3

While #lad2025s answer is good for two columns, if you wanted to extend the method to more than two it can get a bit cumbersome.
A flexible alternative is:
check ((case when answer_id is null then 0 else 1 end +
case when question_id is null then 0 else 1 end ) = 1)
It extends well to checking for a particular count of null (or non-null) values for an arbitrary number of columns.
For example, if you had column_1, column_2, column3, and column_4, and wanted at least 1 of them to be non-null, then:
check ((case when column_1 is null then 0 else 1 end +
case when column_2 is null then 0 else 1 end +
case when column_3 is null then 0 else 1 end +
case when column_4 is null then 0 else 1 end ) >= 1)