SQL> create table test(id int,age int);//创建表

Table created.

SQL> insert into test values(1,10);    //填充数据

1 row created.

SQL> insert into test values(2,20);

1 row created.

SQL> select * from test;               //实验前，表的内容

        ID        AGE
---------- ----------
         1         10
         2         20

SQL> alter table test                  //添加一列name,并设置默认值为'I love oracle'
  2  add name varchar(20) default 'I love oracle';

Table altered.

SQL> select * from test;               //可以看到，设置过后，表中前两行不仅多了一列name，
                                       //而且值也被设置成了'I love oracle'
        ID        AGE NAME
---------- ---------- --------------------
         1         10 I love oracle
         2         20 I love oracle

SQL> alter table test                   //再添加一列city
  2  add city char(10);

Table altered.

SQL> select * from test;

        ID        AGE NAME                 CITY
---------- ---------- -------------------- ----------
         1         10 I love oracle
         2         20 I love oracle

SQL> alter table test                    //设置默认值为'nanjing'
  2  modify city default 'nanjing';

Table altered.

SQL> select * from test;                 //查看后，发现前两行记录并未受到影响

        ID        AGE NAME                 CITY
---------- ---------- -------------------- ----------
         1         10 I love oracle
         2         20 I love oracle

SQL> insert into test(id,age) values(3,30);//添加第3条记录

1 row created.

SQL> select * from test;                   //查看发现，第3条记录的city值被设置成了'nanjing'

        ID        AGE NAME                 CITY
---------- ---------- -------------------- ----------
         1         10 I love oracle
         2         20 I love oracle
         3         30 I love oracle        nanjing

总结：alter table xxx add xxx default xxx，对语句前后的新旧记录都有效
      而alter table xxx add xxx;
      再alter table xxx modify xxx default xxx,只对语句后记录有效