NO.3 引用完整性

定义外键约束：FOREIGN KEY

        CREATE TABLE stu(

                sid int pirmary key,

                name varchar(50) not null,

        );    

        CREATE TABLE score1(

                   score int,

                   sid int , --外键列的数据类型一定要与主键的类型一致

                   CONSTRAINT fk_score1_sid foreign key (sid)  references stu(id)

        );