--創建表(非空、唯一、主鍵、外鍵、check)
create table backtable as select * from oldtable;--備份表
create table tablename(
colname1 varchar2(num),--varchar2不定長,省空間,最多4K,4096B
colname2 char(num),--char定長,效率高
colname3 number(p,s),
colname4 date,
colname5 long,--變長字符串
);
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,--兩個空值不認爲重複
constraint stu_id_pk primary key (id),--非空且唯一,邏輯上唯一代表一條記錄
constraint stu_class_fk foreign key (class) references class(id),--被參考的字段必須是主鍵
constraint stu_name_email_uni unique(name,email)
);
create table class(
id number(4) primary key,
name varchar2(20) not null
);
--修改表結構
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);--有數據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可以查看對應的表創建語句,繼而可以查看系統給無名約束指定的名字
--創建視圖 view
就是一個子查詢,簡化查詢,增加了維護成本,保護私有數據;視圖可以用來更新數據,但是不常用
create view as(
···
)
create view v$_stu as select id,name,age from stu;