1       Managing Schema Objects

1.1              Add and Modify Columns

1.11         Adding Columns

ALTER TABLE CRUISE_ORDERS ADD (ORDER_DATE VARCHAR2(20));

Notice the structure of this statement:

n  The required keywords ALTER TABLE

n  The name of the table we wish to alter

n  The required keyword ADD

n  The name of a new column, and its column specification, enclosed in parentheses

For each column, the clause specifies one or more of the following three elements:

n  Datatype and datatype specification—required

n  DEFAULT and default value—optional

n  CONSTRAINT definition—optional

syntactically wrong:

ALTER TABLE CRUISE_ORDERS

      ADD (WEATHER_CODE DEFAULT 0 NUMBER(2));

But this is syntactically correct:

ALTER TABLE CRUISE_ORDERS

      ADD (WEATHER_CODE NUMBER(2) DEFAULT 0);

1.12         Adding Not Null Columns

如果表已经存在行，不可添加not null的列：

01   ALTER TABLE CRUISE_ORDERS

02         ADD   FIRST_TIME_CUSTOMER VARCHAR2(5)

03               NOT NULL;

设置缺省值才可。//如果添加的列为not null，且原有表有行，必须添加default参数

01   ALTER TABLE CRUISE_ORDERS

02         ADD   FIRST_TIME_CUSTOMER VARCHAR2(5)

03               DEFAULT 'YES'

04               NOT NULL;