Set a column to be nullable

Question

Hi, I am not really good at SQL. Please help me if you can.
I have a column that's Nullable (yes), default (1). Now I want to set the default of the column to null and set the column to be nullable. Here is what I am doing:
create table testit
(foo number default 1 not null, doo varchar(10));
insert into testit (foo, doo) values (15, 'a')
insert into testit (foo, doo) values (14, 'b')
insert into testit (doo) values ('c')
select * from testit
This will return 3 rows where foo=1 when doo='c'. Now I do:
ALTER TABLE testit
modify foo DEFAULT NULL
This will set the default value of foo to be NULL, however, the Nullable is still No. Then when I do
insert into testit (doo) values ('ddd')
I get "cannot insert Null into (testit.foo). What should I do?
Thanks. :D

Answer 1

SQL> create table testit
  2  (foo number default 1 not null, doo varchar(10));
Table created.
SQL> insert into testit (foo, doo) values (15, 'a');
1 row created.
SQL> insert into testit (foo, doo) values (14, 'b');
1 row created.
SQL> insert into testit (doo) values ('c');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from testit;
       FOO DOO
        15 a
        14 b
         1 c
SQL> describe testit;
Name                                      Null?    Type
FOO                                       NOT NULL NUMBER
DOO                                                VARCHAR2(10)
SQL> ALTER TABLE testit
  2  modify foo DEFAULT NULL;
Table altered.
SQL> describe testit;
Name                                      Null?    Type
FOO                                       NOT NULL NUMBER
DOO                                                VARCHAR2(10)
SQL> insert into testit (doo) values ('ddd');
insert into testit (doo) values ('ddd')
ERROR at line 1:
ORA-01400: cannot insert NULL into ("TESTIT"."FOO")
SQL> select constraint_name from all_cons_columns
  2   where table_name = 'TESTIT';
CONSTRAINT_NAME
SYS_C00289436
SQL> select search_condition from all_constraints
  2   where constraint_name =  'SYS_C00289436';
SEARCH_CONDITION
"FOO" IS NOT NULL
SQL> alter table testit drop constraint SYS_C00289436;
Table altered.
SQL> insert into testit (doo) values ('ddd');
1 row created.
SQL> commit;
Commit complete.
SQL>