**约束条件：**
FOREIGN KEY：在子表中定义一个级表约束
REFERENCES：指定表和父表中的列
ON DELETE CASCADE：当删除父表时，级联删除子表纪录
ON DELETE SET NULL：将子表的相关依赖纪录外键置为 null
create table teacher ( sid number constraint teacher_sid_pk primary key, sname varchar2(20) constraint teacher_sname_notnull not null, email varchar2(64) constraint teacher_email_notnull not null constraint teacher_email_unique unique, sex number constraint teacher_sex_min_check check(sex in(1,2)), age number constraint teacher_age check(age>20) );
**插入:**
insert into teacher(sid, sname, email, sex, age) values (1, ‘tom11‘, ‘11@qq.com‘, 1, 21);
# 主外键约束
create table teacher2 ( sid number constraint teacher2_sid_pk primary key, sname varchar2(20) constraint teacher2_sname_notnull not null, email varchar2(64) constraint teache2r_email_notnull not null constraint teache2r_email_unique unique, sex number constraint teacher2_sex_min_check check(sex in(1,2)), age number constraint teacher2_age check(age>20), deptno number constraint teacher2_deptno_pk references dept(deptno) );