mysql实现查询每门课程成绩最好的前两名学生id和姓名

create database db6;

mysql> create table class(cid int not null unique auto_increment,
-> caption varchar(6) not null,
-> grade_id int not null);

mysql> create table student(sid int not null unique auto_increment,
-> sname varchar(3) not null,
-> gender enum(‘male',‘female') not null default ‘male',
-> class_id int not null);

mysql> create table teacher(tid int not null unique auto_increment,
-> tname varchar(6));

mysql> create table course(cid int not null unique auto_increment,
-> cname varchar(4) not null,
-> teacher_id int );

mysql> create table score(sid int not null unique auto_increment,
-> student_id int not null,
-> course_id int not null,
-> score int not null);

mysql> create table class_grade(gid int not null unique auto_increment,
-> gname varchar(6) not null );

mysql> create table teach2cls(tcid int not null unique auto_increment,
-> tid int not null,
-> cid int not null);

insert class (caption,grade_id) values
(‘一年一班',1),(‘一年二班',1),
(‘二年一班',2),
(‘三年一班',3),(‘三年二班',3),(‘三年三班',3),
(‘四年一班',4),(‘四年二班',4),(‘四年三班',4),(‘四年四班',4);

insert student(sname,gender,class_id) values
(‘后羿',‘male',1),(‘盖伦',‘male',2),(‘潘金莲',‘female',1),(‘诸葛亮',‘male',3),
(‘曹操',‘male',3),(‘嬴政',‘male',4),(‘嫦娥',‘female',7),(‘黑老大',‘female',6),
(‘王老二',‘male',5);

insert teacher(tname) values (‘狗蛋'),(‘二锅'),(‘老肥');

insert course(cname,teacher_id) values
(‘语文',1),(‘数学',2),(‘英语',1),(‘物理',3);

insert score(student_id,course_id,score) values
(1,1,61),(1,2,59),(1,3,10),
(2,4,80),(2,3,25),
(3,1,90),(3,2,33),(3,3,100),(3,4,16),
(4,1,100),(4,2,100),(4,4,100),
(5,1,95),(5,2,19),(5,3,59),
(6,3,11),(6,2,61),
(7,3,85),(7,4,99),
(8,1,85),
(9,2,100);

insert class_grade(gname) values
(‘一年级'),(‘二年级'),(‘三年级'),(‘四年级');

insert teach2cls (tid,cid) values
(1,1),(1,3),(2,2),(3,4),(2,8),(3,6),(1,7),(3,10),(3,9),(1,5);

select * from score
where (select count(1)
from score as sc
where score.course_id = sc.course_id
and score.score < sc.score)<2
order by course_id,score desc;
#首先count(1)和count(*)都是统计记录的行数

select count(1) from score as sc where score.course_id = sc.course_id and score.score < sc.score

id-------course-------score
1----------4 ---------100
2 ---------5 ------------90
3 --------6 ------------80