Modify a primary key constraint

Question

Hi,
I'm starting with oracle, and I have two doubts that can't handle after searching on the internet.
1º It's possible to make an ALTER TABLE of a CONSTRAINT DEFAULT?. Maybe you think that the question should be in another way like this: Does oracle database take the default condition as a constraint?. As long as I know, doesn't do that.
2º It's possible, with an ALTER TABLE, to MODIFY a CONSTRAINT of a PRIMARY KEY, to add another more field?
I've been searching on the internet, but I didn't find a clear answer. I'm using oracle 10g express edition.
Thank you very much.
P.D.:Sorry for my english it's not very good.

Answer 1

#2 - You can not 'add a column to the primary-key constraint of a table' with a single ALTER, but you can drop and re-create the primary key constraint (including the additional column).
13:25:26> create table t1 (a number primary key, b number, c number );
Table created.
Elapsed: 00:00:00.09
13:25:46> alter table t1 drop primary key;
Table altered.
Elapsed: 00:00:00.06
13:25:47> alter table t1 add primary key (a,b);
Table altered.
Elapsed: 00:00:00.09But you will not be able to drop the primary key of an index-organized table.
13:25:47> create table t2 (a number primary key, b number, c number ) organization index;
Table created.
Elapsed: 00:00:00.03
13:25:59> alter table t2 drop primary key;
alter table t2 drop primary key
ERROR at line 1:
ORA-25188: cannot drop/disable/defer the primary key constraint for
index-organized tables or sorted hash cluster
Elapsed: 00:00:00.00
13:25:59> #1 - I'm not sure I understand your question , but you can alter a table's column to have a default. This is not a 'constraint', but a default. It only has effect when you perform an insert which does not include that column.
13:31:23> alter table t1 modify (c default 3.14);
Table altered.
Elapsed: 00:00:00.04
13:31:24> insert into t1 (a,b,c) values (1,2,3);
1 row created.
Elapsed: 00:00:00.01
13:31:24> insert into t1 (a,b,c) values (4,5,NULL);
1 row created.
Elapsed: 00:00:00.00
13:31:24> insert into t1 (a,b) values(6,7);
1 row created.
Elapsed: 00:00:00.01
13:31:24> select * from t1;
         A          B          C
         1          2          3
         4          5
         6          7       3.14
3 rows selected.
Elapsed: 00:00:00.18