mysql五-2:多表查询

一 介绍

本节主题

多表连接查询

复合条件连接查询

子查询

准备表

company.employee
company.department

#建表
create table department(
id int,
name varchar(20)
);

create table employee(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

#插入数据
insert into department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into employee(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;

#查看表结构和数据
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+

mysql> desc employee;
+--------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| sex | enum('male','female') | NO | | male | |
| age | int(11) | YES | | NULL | |
| dep_id | int(11) | YES | | NULL | |
+--------+-----------------------+------+-----+---------+----------------+

mysql> select * from department;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+

mysql> select * from employee;
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+

二 多表连接查询

#重点：外链接语法

SELECT 字段列表
    FROM 表1 INNER|LEFT|RIGHT JOIN 表2
    ON 表1.字段 = 表2.字段;

1 交叉连接：不适用任何匹配条件。生成笛卡尔积

mysql> select * from employee,department;
+----+------------+--------+------+--------+------+--------------+
| id | name       | sex    | age  | dep_id | id   | name         |
+----+------------+--------+------+--------+------+--------------+
|  1 | egon       | male   |   18 |    200 |  200 | 技术         |
|  1 | egon       | male   |   18 |    200 |  201 | 人力资源     |
|  1 | egon       | male   |   18 |    200 |  202 | 销售         |
|  1 | egon       | male   |   18 |    200 |  203 | 运营         |
|  2 | alex       | female |   48 |    201 |  200 | 技术         |
|  2 | alex       | female |   48 |    201 |  201 | 人力资源     |
|  2 | alex       | female |   48 |    201 |  202 | 销售         |
|  2 | alex       | female |   48 |    201 |  203 | 运营         |
|  3 | wupeiqi    | male   |   38 |    201 |  200 | 技术         |
|  3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |
|  3 | wupeiqi    | male   |   38 |    201 |  202 | 销售         |
|  3 | wupeiqi    | male   |   38 |    201 |  203 | 运营         |
|  4 | yuanhao    | female |   28 |    202 |  200 | 技术         |
|  4 | yuanhao    | female |   28 |    202 |  201 | 人力资源     |
|  4 | yuanhao    | female |   28 |    202 |  202 | 销售         |
|  4 | yuanhao    | female |   28 |    202 |  203 | 运营         |
|  5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
|  5 | liwenzhou  | male   |   18 |    200 |  201 | 人力资源     |
|  5 | liwenzhou  | male   |   18 |    200 |  202 | 销售         |
|  5 | liwenzhou  | male   |   18 |    200 |  203 | 运营         |
|  6 | jingliyang | female |   18 |    204 |  200 | 技术         |
|  6 | jingliyang | female |   18 |    204 |  201 | 人力资源     |
|  6 | jingliyang | female |   18 |    204 |  202 | 销售         |
|  6 | jingliyang | female |   18 |    204 |  203 | 运营         |
+----+------------+--------+------+--------+------+--------------+

2 内连接：只连接匹配的行

#找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果
#department没有204这个部门，因而employee表中关于204这条员工信息没有匹配出来
mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id;
+----+-----------+------+--------+--------------+
| id | name      | age  | sex    | name         |
+----+-----------+------+--------+--------------+
|  1 | egon      |   18 | male   | 技术         |
|  2 | alex      |   48 | female | 人力资源     |
|  3 | wupeiqi   |   38 | male   | 人力资源     |
|  4 | yuanhao   |   28 | female | 销售         |
|  5 | liwenzhou |   18 | male   | 技术         |
+----+-----------+------+--------+--------------+

#上述sql等同于
mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

3 外链接之左连接：优先显示左表全部记录

#以左表为准，即找出所有员工信息，当然包括没有部门的员工
#本质就是：在内连接的基础上增加左边有右边没有的结果
mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;
+----+------------+--------------+
| id | name       | depart_name  |
+----+------------+--------------+
|  1 | egon       | 技术         |
|  5 | liwenzhou  | 技术         |
|  2 | alex       | 人力资源     |
|  3 | wupeiqi    | 人力资源     |
|  4 | yuanhao    | 销售         |
|  6 | jingliyang | NULL         |
+----+------------+--------------+

4 外链接之右连接：优先显示右表全部记录

#以右表为准，即找出所有部门信息，包括没有员工的部门
#本质就是：在内连接的基础上增加右边有左边没有的结果
mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;
+------+-----------+--------------+
| id   | name      | depart_name  |
+------+-----------+--------------+
|    1 | egon      | 技术         |
|    2 | alex      | 人力资源     |
|    3 | wupeiqi   | 人力资源     |
|    4 | yuanhao   | 销售         |
|    5 | liwenzhou | 技术         |
| NULL | NULL      | 运营         |
+------+-----------+--------------+

5 全外连接：显示左右两个表全部记录

#注意 union与union all的区别：union会去掉相同的纪录

全外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
#注意：mysql不支持全外连接 full JOIN
#强调：mysql可以使用此种方式间接实现全外连接
select * from employee left join department on employee.dep_id = department.id
union
select * from employee right join department on employee.dep_id = department.id
;
#查看结果
+------+------------+--------+------+--------+------+--------------+
| id   | name       | sex    | age  | dep_id | id   | name         |
+------+------------+--------+------+--------+------+--------------+
|    1 | egon       | male   |   18 |    200 |  200 | 技术         |
|    5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
|    2 | alex       | female |   48 |    201 |  201 | 人力资源     |
|    3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |
|    4 | yuanhao    | female |   28 |    202 |  202 | 销售         |
|    6 | jingliyang | female |   18 |    204 | NULL | NULL         |
| NULL | NULL       | NULL   | NULL |   NULL |  203 | 运营         |
+------+------------+--------+------+--------+------+--------------+

三 符合条件连接查询

#示例1：以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,即找出公司所有部门中年龄大于25岁的员工
select employee.name,employee.age from employee,department
    where employee.dep_id = department.id
    and age > 25;

#示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显示
select employee.id,employee.name,employee.age,department.name from employee,department
    where employee.dep_id = department.id
    and age > 25
    order by age asc;

四 子查询

#1：子查询是将一个查询语句嵌套在另一个查询语句中。
#2：内层查询语句的查询结果，可以为外层查询语句提供查询条件。
#3：子查询中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
#4：还可以包含比较运算符：= 、 !=、> 、<等

1 带IN关键字的子查询

#查询employee表，但dep_id必须在department表中出现过
select * from employee
    where dep_id in
        (select id from department);

2 带比较运算符的子查询\

#比较运算符：=、!=、>、>=、<、<=、<>
#查询平均年龄在25岁以上的部门名
select id,name from department
    where id in
        (select dep_id from employee group by dep_id having avg(age) > 25);

#查看技术部员工姓名
select name from employee
    where dep_id in
        (select id from department where name='技术');

#查看不足1人的部门名
select name from department
    where id in
        (select dep_id from employee group by dep_id having count(id) <=1);

3 带EXISTS关键字的子查询

EXISTS关字键字表示存在。在使用EXISTS关键字时，内层查询语句不返回查询的记录。
而是返回一个真假值。True或False
当返回True时，外层查询语句将进行查询；当返回值为False时，外层查询语句不进行查询

#department表中存在dept_id=203，Ture
mysql> select * from employee
    ->     where exists
    ->         (select id from department where id=200);
+----+------------+--------+------+--------+
| id | name       | sex    | age  | dep_id |
+----+------------+--------+------+--------+
|  1 | egon       | male   |   18 |    200 |
|  2 | alex       | female |   48 |    201 |
|  3 | wupeiqi    | male   |   38 |    201 |
|  4 | yuanhao    | female |   28 |    202 |
|  5 | liwenzhou  | male   |   18 |    200 |
|  6 | jingliyang | female |   18 |    204 |
+----+------------+--------+------+--------+

#department表中存在dept_id=205，False
mysql> select * from employee
    ->     where exists
    ->         (select id from department where id=204);
Empty set (0.00 sec)

五 综合练习

init.sql文件内容

/*
 数据导入：
 Navicat Premium Data Transfer

 Source Server         : localhost
 Source Server Type    : MySQL
 Source Server Version : 50624
 Source Host           : localhost
 Source Database       : sqlexam

 Target Server Type    : MySQL
 Target Server Version : 50624
 File Encoding         : utf-8

 Date: 10/21/2016 06:46:46 AM
*/

SET NAMES utf8;
SET FOREIGN_KEY_CHECKS = 0;

-- ----------------------------
--  Table structure for `class`
-- ----------------------------
DROP TABLE IF EXISTS `class`;
CREATE TABLE `class` (
  `cid` int(11) NOT NULL AUTO_INCREMENT,
  `caption` varchar(32) NOT NULL,
  PRIMARY KEY (`cid`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

-- ----------------------------
--  Records of `class`
-- ----------------------------
BEGIN;
INSERT INTO `class` VALUES ('', '三年二班'), ('', '三年三班'), ('', '一年二班'), ('', '二年九班');
COMMIT;

-- ----------------------------
--  Table structure for `course`
-- ----------------------------
DROP TABLE IF EXISTS `course`;
CREATE TABLE `course` (
  `cid` int(11) NOT NULL AUTO_INCREMENT,
  `cname` varchar(32) NOT NULL,
  `teacher_id` int(11) NOT NULL,
  PRIMARY KEY (`cid`),
  KEY `fk_course_teacher` (`teacher_id`),
  CONSTRAINT `fk_course_teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`tid`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

-- ----------------------------
--  Records of `course`
-- ----------------------------
BEGIN;
INSERT INTO `course` VALUES ('', '生物', ''), ('', '物理', ''), ('', '体育', ''), ('', '美术', '');
COMMIT;

-- ----------------------------
--  Table structure for `score`
-- ----------------------------
DROP TABLE IF EXISTS `score`;
CREATE TABLE `score` (
  `sid` int(11) NOT NULL AUTO_INCREMENT,
  `student_id` int(11) NOT NULL,
  `course_id` int(11) NOT NULL,
  `num` int(11) NOT NULL,
  PRIMARY KEY (`sid`),
  KEY `fk_score_student` (`student_id`),
  KEY `fk_score_course` (`course_id`),
  CONSTRAINT `fk_score_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`cid`),
  CONSTRAINT `fk_score_student` FOREIGN KEY (`student_id`) REFERENCES `student` (`sid`)
) ENGINE=InnoDB AUTO_INCREMENT=53 DEFAULT CHARSET=utf8;

-- ----------------------------
--  Records of `score`
-- ----------------------------
BEGIN;
INSERT INTO `score` VALUES ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', ''), ('', '', '', '');
COMMIT;

-- ----------------------------
--  Table structure for `student`
-- ----------------------------
DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
  `sid` int(11) NOT NULL AUTO_INCREMENT,
  `gender` char(1) NOT NULL,
  `class_id` int(11) NOT NULL,
  `sname` varchar(32) NOT NULL,
  PRIMARY KEY (`sid`),
  KEY `fk_class` (`class_id`),
  CONSTRAINT `fk_class` FOREIGN KEY (`class_id`) REFERENCES `class` (`cid`)
) ENGINE=InnoDB AUTO_INCREMENT=17 DEFAULT CHARSET=utf8;

-- ----------------------------
--  Records of `student`
-- ----------------------------
BEGIN;
INSERT INTO `student` VALUES ('', '男', '', '理解'), ('', '女', '', '钢蛋'), ('', '男', '', '张三'), ('', '男', '', '张一'), ('', '女', '', '张二'), ('', '男', '', '张四'), ('', '女', '', '铁锤'), ('', '男', '', '李三'), ('', '男', '', '李一'), ('', '女', '', '李二'), ('', '男', '', '李四'), ('', '女', '', '如花'), ('', '男', '', '刘三'), ('', '男', '', '刘一'), ('', '女', '', '刘二'), ('', '男', '', '刘四');
COMMIT;

-- ----------------------------
--  Table structure for `teacher`
-- ----------------------------
DROP TABLE IF EXISTS `teacher`;
CREATE TABLE `teacher` (
  `tid` int(11) NOT NULL AUTO_INCREMENT,
  `tname` varchar(32) NOT NULL,
  PRIMARY KEY (`tid`)
) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

-- ----------------------------
--  Records of `teacher`
-- ----------------------------
BEGIN;
INSERT INTO `teacher` VALUES ('', '张磊老师'), ('', '李平老师'), ('', '刘海燕老师'), ('', '朱云海老师'), ('', '李杰老师');
COMMIT;

SET FOREIGN_KEY_CHECKS = 1;

从init.sql文件中导入数据

#准备表、记录
mysql> create database db1;
mysql> use db1;
mysql> source /root/init.sql

！！！重中之重：练习之前务必搞清楚sql逻辑查询语句的执行顺序

链接：http://www.cnblogs.com/linhaifeng/articles/7372774.html

1、查询所有的课程的名称以及对应的任课老师姓名

2、查询学生表中男女生各有多少人

3、查询物理成绩等于100的学生的姓名

4、查询平均成绩大于八十分的同学的姓名和平均成绩

5、查询所有学生的学号，姓名，选课数，总成绩

6、 查询姓李老师的个数

7、 查询没有报李平老师课的学生姓名

8、 查询物理课程比生物课程高的学生的学号

9、 查询没有同时选修物理课程和体育课程的学生姓名

10、查询挂科超过两门(包括两门)的学生姓名和班级
、查询选修了所有课程的学生姓名

12、查询李平老师教的课程的所有成绩记录

13、查询全部学生都选修了的课程号和课程名

14、查询每门课程被选修的次数

15、查询之选修了一门课程的学生姓名和学号

16、查询所有学生考出的成绩并按从高到低排序（成绩去重）

17、查询平均成绩大于85的学生姓名和平均成绩

18、查询生物成绩不及格的学生姓名和对应生物分数

19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)平均成绩最高的学生姓名

20、查询每门课程成绩最好的前两名学生姓名

21、查询不同课程但成绩相同的学号，课程号，成绩

22、查询没学过“叶平”老师课程的学生姓名以及选修的课程名称；

23、查询所有选修了学号为1的同学选修过的一门或者多门课程的同学学号和姓名；

24、任课最多的老师中学生单科成绩最高的学生姓名

题目

1、查询所有的课程的名称以及对应的任课老师姓名
SELECT c.cname,t.tname FROM course c LEFT JOIN teacher t ON c.teacher_id=t.tid 

2、查询学生表中男女生各有多少人

SELECT gender,COUNT(sid) FROM student GROUP BY gender

3、查询物理成绩等于100的学生的姓名
SELECT stu.sname FROM student stu LEFT JOIN  score s ON s.student_id=stu.sid LEFT JOIN
course c ON c.cid=s.course_id WHERE s.num=100 AND c.cname="物理"

SELECT sname FROM student WHERE sid IN(SELECT student_id FROM score WHERE
num=100 AND course_id =(SELECT cid FROM course WHERE cname="物理"))

4、查询平均成绩大于八十分的同学的姓名和平均成绩
SELECT s.sname, 平均成绩 FROM student s RIGHT JOIN
(SELECT student_id ,AVG(num) 平均成绩 FROM score GROUP BY student_id HAVING AVG(num)>80 )AS sc
ON s.sid=sc.student_id

5、查询所有学生的学号，姓名，选课数，总成绩
SELECT s.sid 学号,s.sname 姓名,选课数,总成绩 FROM student s RIGHT JOIN
(SELECT student_id,COUNT(course_id) 选课数,SUM(num) 总成绩 FROM score
GROUP BY student_id)AS sc
ON s.sid=sc.student_id

6、 查询姓李老师的个数
SELECT COUNT(tid) FROM  teacher WHERE tname LIKE "李%"

7、 查询没有报李平老师课的学生姓名
SELECT sname FROM student WHERE sid  NOT IN
(SELECT student_id FROM score WHERE course_id IN
(SELECT cid FROM course WHERE teacher_id =(SELECT tid FROM teacher WHERE tname="李平老师")))

SELECT sname FROM student WHERE sid NOT IN (
SELECT student_id FROM score s LEFT JOIN course c ON c.cid=s.course_id WHERE c.cid IN(
SELECT cid FROM course c INNER JOIN teacher t ON t.tid=c.teacher_id WHERE t.tname="李平老师"
)
)
8、查询物理课程比生物课程高的学生的学号
SELECT * FROM
(SELECT  student_id 学生号, num 成绩,cname 课程 FROM score LEFT JOIN course ON course_id=score.course_id WHERE cname="物理")AS wl
 INNER JOIN
(SELECT  student_id 学生号, num 成绩,cname 课程 FROM score LEFT JOIN course ON course_id=score.course_id WHERE cname="生物")AS sw
 ON wl.成绩>sw.成绩

 SELECT t1.student_id FROM
(SELECT student_id,num FROM score s INNER JOIN course c ON c.cid=s.course_id WHERE c.cname="物理")AS t1
JOIN
(SELECT student_id,num FROM score s INNER JOIN course c ON c.cid=s.course_id WHERE c.cname="生物")AS t2 ON
t1.student_id=t2.student_id WHERE t1.num>t2.num

9、查询没有同时选修物理课程和体育课程的学生姓名
SELECT sname FROM student WHERE sid NOT IN(
SELECT student_id FROM score WHERE course_id IN (SELECT cid FROM course
 WHERE cname IN ("物理","生物"))GROUP BY student_id  HAVING COUNT(course_id)=2
 )
10、查询挂科超过两门(包括两门)的学生姓名和班级
SELECT sname,cl.caption FROM student stu ,class cl WHERE stu.sid IN(
SELECT s.student_id FROM score s LEFT JOIN course c ON s.course_id = c.cid WHERE num<60 GROUP BY student_id HAVING COUNT(sid)>=2
)AND stu.class_id=cl.cid

SELECT sname,caption FROM student stu ,class cl WHERE stu.sid IN(
  SELECT student_id FROM score WHERE num<60 GROUP BY student_id HAVING COUNT(sid)>=2)
AND stu.class_id=cl.cid

SELECT t2.sname ,cl.caption FROM (
SELECT sname ,class_id FROM student stu INNER JOIN
( SELECT student_id  FROM score WHERE num<60 GROUP BY student_id HAVING COUNT(course_id)>=2)
AS t1 ON stu.sid=t1.student_id
)AS t2 INNER JOIN class cl  ON  cl.cid=t2.class_id

11 、查询选修了所有课程的学生姓名

SELECT s.sid, s.sname FROM student s WHERE s.sid IN(
SELECT s.student_id  FROM score s GROUP BY student_id HAVING COUNT(s.sid)=(SELECT COUNT(cid) FROM course)
)

12、查询李平老师教的课程的所有成绩记录
SELECT s.num ,c.cname FROM score s INNER JOIN course c  ON s.course_id=c.cid WHERE c.cid IN
(SELECT c.cid FROM course c  INNER JOIN  teacher t ON c.teacher_id=t.tid WHERE tname="李平老师")

SELECT s.num ,c.cname FROM score s LEFT JOIN  course c ON c.cid=s.course_id LEFT
JOIN teacher t ON t.tid=c.teacher_id WHERE t.tname="李平老师"

13、查询全部学生都选修了的课程号和课程名
SELECT cid,cname FROM course WHERE cid IN(
SELECT course_id FROM score GROUP BY course_id  HAVING COUNT(sid)=(
SELECT COUNT(sid) FROM student
)
)

SELECT c.cid,c.cname FROM course c WHERE c.cid IN(
SELECT course_id FROM score GROUP BY course_id HAVING COUNT(sid)=(SELECT COUNT(sid) FROM student)
)
14、查询每门课程被选修的次数 

SELECT c.cname ,COUNT(sid) FROM course c LEFT JOIN score s ON  s.course_id =c.cid GROUP BY s.course_id

15、查询只选修了一门课程的学生姓名和学号
SELECT sid,sname FROM student WHERE sid IN(SELECT student_id FROM score GROUP BY student_id HAVING COUNT(sid)=1)

16、查询所有学生考出的成绩并按从高到低排序（成绩去重）
查询每个考生对应的成绩
SELECT stu.sname,s.num FROM student stu LEFT JOIN score s ON s.student_id=stu.sid ORDER BY s.num DESC
查询所有学生考出的成绩并按从高到低排序（成绩去重）
SELECT DISTINCT num FROM score ORDER BY num DESC

17查询平均成绩大于85的学生姓名和平均成绩

SELECT stu.sname 姓名,AVG(s.num )平均成绩 FROM student stu INNER JOIN score s ON
stu.sid =s.student_id GROUP BY s.student_id HAVING AVG(num)>85 

18、查询生物成绩不及格的学生姓名和对应生物分数

SELECT stu.sname 姓名,s.num 生物成绩 FROM score s
LEFT JOIN student stu  ON stu.sid=s.student_id
LEFT JOIN course c ON s.course_id=c.cid
WHERE c.cname="生物" AND s.num<60

19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)
平均成绩最高的学生姓名

SELECT stu.sname FROM score s LEFT JOIN student stu ON s.student_id=stu.sid
LEFT JOIN course c ON c.cid=s.course_id WHERE c.cid IN (SELECT c.cid FROM course c LEFT JOIN
teacher t ON c.teacher_id =t.tid WHERE t.tname="李平老师"
) GROUP BY student_id ORDER BY AVG(s.num) DESC LIMIT 1

20、查询每门课程成绩最好的前两名学生姓名

 SELECT s.sid,s.course_id,s.num,ss.first_num,ss.second_num FROM score s LEFT JOIN
    (
    SELECT
        sid,
        (SELECT num FROM score AS s2 WHERE s2.course_id = s1.course_id ORDER BY num DESC LIMIT 0,1) AS first_num,
        (SELECT num FROM score AS s2 WHERE s2.course_id = s1.course_id ORDER BY num DESC LIMIT 1,1) AS second_num
    FROM
        score AS s1
    ) AS ss
    ON s.sid =ss.sid
    WHERE s.num <= ss.first_num AND s.num >= ss.second_num

21、查询不同课程但成绩相同的学号，课程号，成绩

SELECT  s1.student_id,  s1.course_id, s1.num, s2.student_id , s2.course_id,s2.num FROM score AS s1,
score AS s2 WHERE s1.num = s2.num AND s1.course_id != s2.course_id AND s1.student_id!=
s2.student_id;

22、查询没学过“叶平”老师课程的学生姓名以及选修的课程名称；

SELECT stu.sname 姓名 FROM student stu WHERE stu.sid NOT IN (
SELECT student_id FROM score WHERE course_id IN(
SELECT cid FROM course WHERE teacher_id=(
SELECT tid FROM teacher WHERE tname="李平老师"
)
)
)

23、查询所有选修了学号为1的同学选修过的一门或者多门课程的同学学号和姓名；
SELECT DISTINCT stu.sid, stu.sname FROM student stu LEFT JOIN score s ON  s.student_id=stu.sid WHERE s.course_id IN(
SELECT course_id FROM score WHERE student_id =1)

SELECT stu.sid,stu.sname  FROM student stu WHERE stu.sid IN(
SELECT student_id FROM score WHERE course_id IN (SELECT course_id FROM score WHERE student_id =1)
)

24、任课最多的老师中学生单科成绩最高的学生姓名
SELECT stu.sname FROM score s LEFT JOIN  student stu ON stu.sid=s.student_id
LEFT JOIN course c ON s.course_id =c.cid WHERE c.teacher_id =(
SELECT teacher_id FROM course  GROUP BY teacher_id ORDER BY COUNT(cid) DESC LIMIT 1)
GROUP BY s.student_id  ORDER BY AVG(s.num) DESC LIMIT 2

SELECT stu.sname FROM student stu LEFT JOIN score s ON s.student_id =stu.sid
LEFT JOIN course c ON c.cid=s.course_id WHERE cid IN( SELECT cid FROM course WHERE teacher_id =(SELECT teacher_id FROM course
GROUP BY teacher_id ORDER BY COUNT(cid) DESC LIMIT 1))GROUP BY s.student_id  ORDER BY AVG(s.num) DESC LIMIT 2