oracle
主键约束主键约束列级：create table types(t_id number primary key,t_name varchar2(30) unique,t_description varchar2(100),constraint types_unique_name unique(t_name) );主键约束表级：create table types(t_id number(4),t_name varchar2(30),t_description varchar2(100),constraint types_pk_id primary key(t_id),constraint types_unique_name unique(t_name));
外键约束外键约束表级：create table goods(g_id number(4) primary key,g_name varchar2(30) unique,t_id number(4),g_desc varchar2(200),constraint goods_pk_tid foreign key (t_id) references types(t_id));外键约束列级：create table goods1(g_id number(4) primary key,g_name varchar2(30) unique,t_id number(4) references types(t_id),g_desc varchar2(200));
唯一约束唯一约束表级：create table types(t_id number not null,t_name varchar2(30),t_description varchar2(100),constraint types_unique_name unique(t_name) );唯一约束列级：create table types(t_id number not null,t_name varchar2(30) unique,t_description varchar2(100) );
约束条件检查约束： create table students( s_id varchar2(10) primary key, s_name varchar2(20), c_id varchar2(20), s_sex char(2) check(s_sex='男' or s_sex='女')， s_age number(3) check(s_age>20 and s_age<150), constraint students_fk_id foreign key(c_id) references class(c_id)) ; create table class( c_id varchar2(20) primary key, c_name varchar2(20) );insert into class values(1,'男');insert into students values(1,'强强'，1,'男'，21);
修改约束： create table classes1( c_id varchar2(20) primary key, c_name varchar2(20) );create table stud(stu_id number(4) primary key,stu_name varchar2(20) not null,c_id number references classes1(c_id),stu_sex char(2) check(stu_sex in('男','女')));create table stud1(stu_id number(4) primary key,stu_name varchar2(20) not null,c_id number references classesl(c_id),stu_sex char(2) check (stu_sex='男' or stu_sex='女'));添加约束：create table stud2(stu_id number(4),stu_name varchar2(20) not null,c_id number references classesl(c_id),stu_sex char(2) check (stu_sex='男' or stu_sex='女'));建表后添加主键约束：alter table stud2 add constraint stud2_pk_id primary key(stu_id);建表后添加非空约束：alter table stud2 modify stu_sex not null;