Question

create table t1(ider number null);  

Does it behave as:

create table t1(ider number check (ider is null));  

or

create table t1(ider number default null);  

?

Answer 1

None of your two hypotheses.

Oracle takes NULL in a column specification as the explicit absence of a NOT NULL constraint. It is a (misleading) no-op in a CREATE TABLE statement, but not necessarily in an ALTER TABLE statement where it can revoke a previously existing constraint.

CREATE TABLE t(col1 TYPE NOT NULL);
ALTER TABLE t MODIFY col1 NULL; -- Now the `NOT NULL` constraint has been dropped

Therefore when you use CREATE TABLE t(c INTEGER NULL) it does not mean that c should be always null, nor does it mean that c has default value NULL. It just means that c being null is ok.