Default can be considered as a substitute behavior of not null constraint when applied to new
rows being entered into the table.
When you
define a column with the default
keyword followed by a value, you are actually telling the database that, on
insert if a row was not assigned a value for this column, use the default value
that you have specified.
Default is
applied only during insertion of new rows.
Ex:
SQL> create table student12(no number(2)
default 11,name varchar(2));
SQL> insert into student12 values(1,'a');
SQL> insert into student12(name) values('b');
SQL> select * from student12;
NO
NAME
------ ---------
1 a
11 b
SQL> insert into student12 values(null, ‘c’);
SQL> select * from student12;
NO
NAME
------ ---------
1 a
11 b
C
-- Default can
not override nulls.
