So i have two nested tables and i want to make a new one with the elements from both of them but the first nested table have an null value and the second one an number and i want the result to be the number in the second one but he print the null value. It is possible to make a union between a null and an number with multiset union ?
To answer your question, yes, it is possible to "make a union between a null and an number with multiset union". But what you end up with is **two entries in the nested table:
SQL> update test
2 set marks = numberlist(null) multiset union all numberlist(42)
3 where id_std = 1
4 /
SQL> select id_std
2 , t2.column_value as mark
3 from test t1
4 , table(t1.marks) t2
5 /
ID_STD MARK
------ ----
1
1 42
SQL>
I suspect this affect is actually what you're complaining about. However, the null mark is still a valid entry. If you want to overwrite it you need to provide different logic.