Adding a default constraint in Oracle while altering a column
A default value is not a constraint in Oracle. You simply alter the column as:

SQL&gt; create table tab1 (col1 number, col2 number);

Table created.

SQL&gt; alter table tab1 modify (col2 default 1);

Table altered.

SQL&gt; select * from user_constraints;

no rows selected

SQL&gt; insert into tab1 (col1) values (0);

1 row created.

SQL&gt; select * from tab1;

      COL1       COL2
---------- ----------
         0          1