drop table example;
 CREATE TABLE example(
     ID Number(4) NOT NULL ,
     NAME VARCHAR(25),
     constraint example_id primary key(id)
  );
drop SEQUENCE example_sequence;
CREATE SEQUENCE example_sequence
INCREMENT BY 1 -- 每次加几个
START WITH 1 -- 从1开始计数
NOMAXVALUE -- 不设置最大值
NOCYCLE -- 一直累加，不循环
NOCACHE -- 不建缓冲区
;
drop TRIGGER example_triger;
CREATE TRIGGER example_triger
  BEFORE INSERT ON example
  FOR EACH ROW
  WHEN (new.id is null) --只有在id为空时，启动该触发器生成id号
begin
  select example_sequence.nextval into :new.id from dual;
end;

insert into example(name) values('张三');
insert into example(id, name) values(111,'李四');
insert into example(name) values('张三1');
insert into example(name) values('张三2');
insert into example(name) values('张三3');

select * from example;
drop table xxx;
create table xxx as select * from example; --复制一个表的数据和结构，但约束不会被复制
select * from xxx;

--修改主键
--1查主键
 SELECT   *   from   user_cons_columns c where c.table_name = 'example';
 SELECT   *   from   user_cons_columns c where c.table_name = 'xxx';
--2删除主键
   alter table  example drop constraint example_id;
--3增加新的主键约束  
   alter table example add constraint example_id primary key(id);
--4添加列
   alter table example add age number;
--5删除列
   alter table example drop column age;
--6修改列(修改列类型的前提：需删除存在的数据库)
   alter table example modify age varchar(2);
   alter table example modify id varchar(20);
--7添加约束
 -- foreign key  
  alter table example add constraint fk_example_xxx foreign key(pid) references xxx(pid);     
   
insert into example(name,age) values('张三1',1);
insert into example(name,age) values('张三2',2);
insert into example(name,age) values('张三3',113);
insert into xxx(name,pid) values('张三2',21);
insert into xxx(name,pid) values('张三3',1111);
select * from example;
select * from xxx;
drop table xxx;
create table xxx as select * from example;
select * from xxx;
--alter table xxx add pid number(10);
-----处理复制表约束
alter table xxx rename column id to sid;
alter table xxx add id number;
update   xxx set id=sid;
alter table xxx drop column sid;
-----处理添加字段并为主键
alter table xxx add pid varchar2(20);
update xxx set pid=id;
alter table xxx add constraint xxx_pid primary key(pid);
select * from xxx ;
----为example表添加外键
alter table xxx add constraint fk_xxx foreign key(id) references example(id);
--删除约束
alter table xxx drop constraint fk_example_xxx;


---------处理已有数据的字段类型修改(前4步操作会使表中的约束丢失)
--1重命名字段
alter table example rename column id to sid;
--2添加id字段
alter table example add id varchar2(200);  
--3更新数据
update example set id = sid;
--4删除备份数据的字段
alter table example drop column sid;
--5新增约束
alter table example add constraint exmaple_id primary key(id);
-------------------------------------------------------------------
alter table example add unique(age);
alter table example drop unique(age);
--查找表的唯一性约束（包括名称，构成列）
select column_name
  from user_cons_columns cu, user_constraints au
 where cu.constraint_name = au.constraint_name
   and cu.table_name = 'example';

-----视图(  如果权限不足，grant connect,resource,dba to 你的实例)
create table bbb as select * from example;
CREATE  OR  REPLACE  VIEW  exam_sum_v1
(name,age) 
AS 
SELECT d.name,d.age 
FROM bbb d;
insert into exam_sum_v1(name,age) values('lisii',1221);
insert into exam_sum_v1(name,age) values('lisii',1221);
insert into exam_sum_v1(name,age) values('lisii',0000);
insert into exam_sum_v1(name,age) values('lisii',111);
select * from exam_sum_v1;
sele