/*学生*/

create table student(

sno int not null primary key,

sname varchar(10)

);

/*课程*/

create table center(

cno int not null primary key,

cname varchar(10)

);

/*分数*/

create table sgrade(

sno int ,

cno int ,

sgrade int

);

insert into student values(11,'a');

insert into student values(12,'b');

insert into student values(13,'c')

insert into center values(21,'aa');

insert into center values(22,'bb');

insert into center values(23,'cc')

insert into sgrade values(11,21,54);

insert into sgrade values(12,22,57);

insert into sgrade values(11,21,51);

insert into sgrade values(12,21,36);

insert into sgrade values(11,21,28);

insert into sgrade values(12,22,42);

insert into sgrade values(11,21,59)

insert into sgrade values(11,21,79);

insert into sgrade values(12,22,85);

insert into sgrade values(11,21,90);

insert into sgrade values(12,21,96);

insert into sgrade values(11,21,98);

insert into sgrade values(12,22,94);

insert into sgrade values(11,21,99)

SELECT s.sno,s.sname

    from student s

    LEFT JOIN sgrade g on s.sno=g.sno

WHERE g.sgrade<60

GROUP BY s.sno HAVING COUNT(s.sno)>2

SELECT * FROM student WHERE sno not in (

    SELECT s.sno

        from student s

        LEFT JOIN sgrade g on s.sno=g.sno

    WHERE g.sgrade<60

)