新建表/约束时的试验

create table tb_user(
        userid number(5),
        username varchar2(60) constraint nn_user_name not null,
        userpwd varchar2(60) constraint nn_user_pwd not null,
        age number(3) default(18),
        gender char(3) default('男'),
        email varchar2(30),
        regtime data default(sysdate),
        constraint pk_user_id primary key (userid),
        constraint ck_user_name check(length(username) between 4 and 20),
        constraint ck_user_pwd check(length(userpwd) between 4 and 18),
        constraint ck_user_age check(age>=18),
        constraint ck_user_gender check(gender in('男','女')),
        constraint uq_user_email unique(email)
    );
    
create table tb_txt(
        txtid number(10),
        title varchar2(60) constraint nn_txt_title not null,
        txt varchar2(1024),
        pubtime data default(sysdate),
        userid number(5),
        constraint pk_txt_id primary key(txtid),
        constraint ck_txt_id check(length(title)>=4 and length(title)<=30),
        constraint fk_txt_ref_user_id foreign key(userid) references tb_user(userid)
on delete set null
    );