Oracle增加修改删除字段/主键

alter table xgj  rename column  old_name to new_name;

alter table tablename modify (column datatype [default value][null/not null],….);

create table xgj(
id number(9) ,
name char(20)
)

SQL> select  * from xgj ;

        ID NAME
---------- --------------------

SQL> alter table xgj modify(name varchar2(20));

Table altered

SQL> 

--紧接着第一个情况操作，将name的类型改为创建时的char(20)
SQL> alter table xgj modify(name char(20));

Table altered
--插入数据
SQL> insert into xgj(id,name) values (1,'xiaogongjiang');

1 row inserted

SQL> select　* from xgj;

        ID NAME
---------- --------------------
         1 xiaogongjiang

 SQL> alter table xgj modify(name varchar2(20));

Table altered

SQL> desc xgj;
Name Type    Nullable Default Comments 
---- ------------ -------- ------- -------- 
ID NUMBER(9) Y                        
NAME VARCHAR2(20) Y                        

SQL> alter table xgj modify(name varchar2(40));
Table altered

SQL> alter table xgj modify(name char(20));
Table altered

--建表
create table xgj (col1 number, col2 number) ;
--插入数据
insert into  xgj(col1,col2) values (1,2);
--提交
commit ;
--修改col1 由number改为varchar2类型 （不兼容的类型）
alter table xgj modify ( col1 varchar2(20))

alter table xgj rename column col1  to col1_tmp;

alter table xgj add col1 varchar2(20);

update xgj  set col1=trim(col1_tmp);

alter table xgj drop column col1_tmp;

alter table tablename add (column datatype [default value][null/not null],….);

alter table xgj

add (name varchar2(30)  default ‘无名氏’ not null,

age integer default 22 not null,

salary number(9,2)

);

alter table tablename drop (column);

create table student (
studentid int primary key not null,
studentname varchar(8),
age int);

create table student (
studentid int primary key not null,
studentname varchar(8),
age int);

create table students (
studentid int ,
studentname varchar(8),
age int,
constraint yy primary key(studentid));

alter table student drop constraint SYS_C002715;

alter table students drop constraint yy;

alter table student add constraint pk_student primary key(studentid);