案例说明：
SQL> create table t(a number);
Table created
SQL> desc t;
Name Type Nullable Default Comments
---- ------ -------- ------- --------
A NUMBER Y
SQL> alter table t add (b number,c number);
Table altered
SQL> desc t;
Name Type Nullable Default Comments
---- ------ -------- ------- --------
A NUMBER Y
B NUMBER Y
C NUMBER Y
SQL> alter table t modify (b varchar2(10),c varchar2(10));
Table altered
SQL> desc t;
Name Type Nullable Default Comments
---- ------------ -------- ------- --------
A NUMBER Y
B VARCHAR2(10) Y
C VARCHAR2(10) Y
SQL> alter table t drop (b,c);
Table altered
SQL> desc t;
Name Type Nullable Default Comments
---- ------ -------- ------- --------
A NUMBER Y