mysql 8.0版本中添加表级约束外键失败原因之一

 1 create table if not exists t_classes(
 2     classes_id int(3),
 3     classes_name varchar(40),
 4     constraint pk_classes_id primary key(classes_id)
 5 );
 6 
 7 -- 利用标记约束添加主键和外键
 8 create table if not exists t_student(
 9     student_id       int(10),
10     student_name     varchar(20),
11     sex           char(2),
12     birthday      date,
13     email         varchar(30),
14     classes_id    int(3),
15     constraint    fk_classes_id foreign key (classes_id) references t_classes (classes_id),
16     constraint    student_id_pk primary key(student_id) 
17 );

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), 
--         这里的添加顺序是先添加主键后添加外键，会失败，提示语法错误
    constraint    student_id_pk primary key(student_id)     
    constraint    fk_classes_id foreign key (classes_id2) references t_classes (classes_id)  
);

同样是添加主键和外键，顺序颠倒后会报语法错误

数据库版本号：8.0.19