DDL（Data Definition Language）语句 on Oracle

--创建表（非空、唯一、主键、外键、check）

create table backtable as select * from oldtable;--备份表

create table tablename(
  colname1 varchar2(num),--varchar2不定长，省空间，最多4K，4096B
  colname2 char(num),--char定长，效率高
  colname3 number(p,s),
  colname4 date,
  colname5 long,--变长字符串 
);

create table stu(
  id number(6),
  name varchar2(20) constraint stu_name_nn not null,
  sex number(1),
  age number(3),
  sdate date,
  grade number(2) default 1,
  class number(4),
  email varchar2(50) constraint stu_email_uni unique,--两个空值不认为重复
  constraint stu_id_pk primary key (id),--非空且唯一，逻辑上唯一代表一条记录
  constraint stu_class_fk foreign key (class) references class(id),--被参考的字段必须是主键
  constraint stu_name_email_uni unique(name,email)
);

create table class(
  id number(4) primary key,
  name varchar2(20) not null
);

--修改表结构

alter table tablename add(cloname coltype);
alter table stu add(addr varcher2(100));

alter table tablename drop (cloname);
alter table stu drop (addr);

alter table tablename modify(cloname type);--有数据了再更改精度不能降低
alter table stu modify (addr varchar2(50));

alter table tablename drop constraint constraintname;
alter table stu drop constraint stu_class_fk;

alter table tablename add constraint constraintname foreign key (colname) references tablename (colname)
alter table stu add constraint stu_class_fk foreign key (class) references class (id);

drop table tablename;

mysql中show create table tablename可以查看对应的表创建语句，继而可以查看系统给无名约束指定的名字

--创建视图 view

就是一个子查询，简化查询，增加了维护成本，保护私有数据;视图可以用来更新数据，但是不常用

create view as(
  ···
)
create view v$_stu as select id,name,age from stu;