mysql 8.0版本中添加表級約束外鍵失敗原因之一

create table if not exists t_classes(
    classes_id int(3),
    classes_name varchar(40),
    constraint pk_classes_id primary key(classes_id)
);

-- 利用標記約束添加主鍵和外鍵
create table if not exists t_student(
    student_id       int(10),
    student_name     varchar(20),
    sex           char(2),
    birthday      date,
    email         varchar(30),
    classes_id    int(3),
    constraint    fk_classes_id foreign key (classes_id) references t_classes (classes_id),
    constraint    student_id_pk primary key(student_id) 
);

create table if not exists t_student(
    student_id       int(10)  primary key,
    student_name     varchar(20),
    sex           char(2),
    birthday      DATE,
    email         varchar(30),
    classes_id2    int(3), 
--         這里的添加順序是先添加主鍵后添加外鍵，會失敗，提示語法錯誤
    constraint    student_id_pk primary key(student_id)     
    constraint    fk_classes_id foreign key (classes_id2) references t_classes (classes_id)  
);

同樣是添加主鍵和外鍵，順序顛倒后會報語法錯誤

數據庫版本號：8.0.19