Oracle identity column and insert into select

I believe the below query works, i havent tested!

INSERT INTO Test (b)SELECT * FROM(   SELECT 'z' FROM dual   UNION ALL   SELECT 'zz' FROM dual);

Not sure, if it helps you any way.

For, GENERATED ALWAYS AS IDENTITY Oracle internally uses a Sequence only. And the options on general Sequence applies on this as well.

NEXTVAL is used to fetch the next available sequence, and obviously it is a pseudocolumn.

The below is from Oracle

You cannot use CURRVAL and NEXTVAL in the following constructs:

• A subquery in a DELETE, SELECT, or UPDATE statement
• A query of a view or of a materialized view
• A SELECT statement with the DISTINCT operator
• A SELECT statement with a GROUP BY clause or ORDER BY clause
• A SELECT statement that is combined with another SELECT statement with the UNION, INTERSECT, or MINUS set operator
• The WHERE clause of a SELECT statement
• DEFAULT value of a column in a CREATE TABLE or ALTER TABLE statement
• The condition of a CHECK constraint

The subquery and SET operations rule above should answer your Question.

And for the reason for NULL, when pseudocolumn(eg. NEXTVAL) is used with a SET operation or any other rules mentioned above, the output is NULL, as Oracle couldnt extract them in effect with combining multiple selects.

Let us see the below query,

select rownum from dualunion all select rownum from dual

the result is

ROWNUM11