方法1：在列的级别上定义约束

create table product (

Pname varchar2(20) unique,

Pdate date not null,

PID number(5) primary key,check(PID>10000 AND PID<=99999),

Ploc char(30) default '北京')

要点就是在列的结尾直接添加，如果同一列有多个约束，通过逗号来分隔

有个问题，通过数字字典来查询约束，我们得到的是系统提供的名字并且约束的类型不好区分。

SQL> select constraint_name,constraint_type from user_constraints where table_name = 'PRODUCT';

CONSTRAINT_NAME                                         CONSTRAINT_TYPE

------------------------------------------------------------ --

SYS_C009539                                                  C

SYS_C009540                                                  C

SYS_C009541                                                  P

SYS_C009542                                                  U

U:表示unique

P:primary key

C:check或者default

查看列的默认值：

SQL> select column_name,datA_dEfault from useR_tab_columns where table_name = 'PRODUCT';

COLUMN_NAME                                                  DATA_DEFAULT

------------------------------------------------------------ --------------------------------------------------------------------------------

PNAME

PDATE

PID

PLOC                                                         '北京'

方法二在表级别上定义约束：

Create table product(

PID number(5),

PNAME varchar2(20),

PDATE date constraint NN_PDATE_PRODUCT not null,

PLOC char(30) default ‘北京’,

FKDEPTNO number(5),

Constraint UK_PNAME_PRODUCT unique(PNAME),

Constraint PK_PID_PRODUCT primary key(PID),

Constraint CK_PID_PRODUCT check(PID>10000 AND PID<9999),

Constraint FK_DEPTNO foreign key (FKDEPTNO) references dept(DEPTNO)

);

SQL> select constraint_name,constraint_type from user_constraints where table_name = 'PRODUCT';

CONSTRAINT_NAME                                              CO

------------------------------------------------------------ --

NN_PDATE_PRODUCT                                             C

CK_PID_PRODUCT                                               C

PK_PID_PRODUCT                                               P

UK_PNAME_PRODUCT                                             U

FK_DEPTNO_PRODUCT                                            R

删除约束

Alter table product drop primary key;

Alter table product drop constraint PK_PID_PRODUCT;

Alter table product drop unique (PNAME);

Alter table product modify(PDATE,null);

Alter table product modify(PLOC,default null)/modify(PLOC ,default ‘威海’);--更改默认值

Alter table product drop constraint FK_DEPTNO;

外键、check约束只能使用约束名来删除，其他约束如果有约束名也可以使用约束名来删除

增加约束

Alter table product add primary key (PID);

Alter table product add constraint PK_PID primary key(PID);

Alter table product add unique(PNAME);

Alter table product modify(PDATE, not null);

Alter table product modify(PLOC ,default ‘威海’);