Modification of table fields by Oracle

Main operation ：1. Add fields ;2. Delete field ;3. Modify fields ( Change the type of field , Modify the length of the field )

1. Add fields ：

You can always succeed , The newly added field appears at the end of the table .

alter table student 

        add tel varchar2(11);

alter table student 

        add addr varchar2(11);

2. Delete field ：

You can always succeed , Whether there is data or not , Can be deleted .

alter table student

        drop column addr

3. Modify fields ：

A. When there is no data in this field , Type of field , The length of the field can be modified .

alter table student  

        modify tel number(11);

alter table student 

        modify addr varchar(20);

B. Modification of default values , It will not affect the existing data , It will only affect the data inserted later .

alter table student

        modify sex char(2) default ' Woman ';

C. When the field has data, the type of the field cannot be modified , But the length of the field can be modified ( The length can be increased , Smaller depends on the actual length of the data ).

alter table student

        modify grade varchar(8);