oracle约束

1.表级约束(主键)
     create table sal_grade_zz(
    sal_grade varchar2(128),
    low_sal number(7,2),
    high_sal number(7,2),
    constraint sal_grade_pk primary key(sal_grade)    
);
 2.列级约束
        create table sal_grade_zz(
    sal_grade varchar2(128)  primary key,
    low_sal number(7,2) not null,
    high_sal number(7,2)
);
3.给列级约束命名
        create table sal_grade_zz(
    sal_grade varchar2(128)  primary key,
    low_sal number(7,2) constraint low_sal_nn not null,
    high_sal number(7,2)
);  
4.唯一性约束(列级约束)
        create table sal_grade_zz(
    sal_grade varchar2(128)  primary key,
    low_sal number(7,2)  not null,
    high_sal number(7,2) unique
        );   
5.表级唯一约束
        create table sal_grade_zz(
    sal_grade varchar2(128)  ,
    low_sal number(7,2) ,
    high_sal number(7,2),