mysql四-2:多表查询一 介绍二 多表连接查询三 符合条件连接查询四 子查询五 综合练习一 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#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.idunionselect * 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 |+------+------------+--------+------+--------+------+--------------+#注意 union与union all的区别：union会去掉相同的纪录三 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,即找出年龄大于25岁的员工以及员工所在的部门select employee.name,department.name from employee inner join department on employee.dep_id = department.id where 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#查询平均年龄在25岁以上的部门名select id,name from department where id in (select dep_id employee group by 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(子查询得到的是有人的部门id)select name from department where id not in (select distinct dep_id from employee);2 带比较运算符的子查询#比较运算符：=、!=、>、>=、<、<=、<>#查询大于所有人平均年龄的员工名与年龄mysql> select name,age from emp where age > (select avg(age) from emp);+---------+------+| name | age |+---------+------+| alex | 48 || wupeiqi | 38 |+---------+------+2 rows in set (0.00 sec)#查询大于部门内平均年龄的员工名、年龄select t1.name,t1.age from emp t1inner join (select dep_id,avg(age) avg_age from emp group by dep_id) t2on t1.dep_id = t2.dep_idwhere t1.age > t2.avg_age; 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，Turemysql> 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，Falsemysql> select * from employee -> where exists -> (select id from department where id=204);Empty set (0.00 sec) 练习：查询每个部门最新入职的那位员工company.employee 员工id id int 姓名 emp_name varchar 性别 sex enum 年龄 age int 入职日期 hire_date date 岗位 post varchar 职位描述 post_comment varchar 薪水 salary double 办公室 office int 部门编号 depart_id int#创建表create table employee(id int not null unique auto_increment,name varchar(20) not null,sex enum('male','female') not null default 'male', #大部分是男的age int(3) unsigned not null default 28,hire_date date not null,post varchar(50),post_comment varchar(100),salary double(15,2),office int, #一个部门一个屋子depart_id int);#查看表结构mysql> desc employee;+--------------+-----------------------+------+-----+---------+----------------+| Field | Type | Null | Key | Default | Extra |+--------------+-----------------------+------+-----+---------+----------------+| id | int(11) | NO | PRI | NULL | auto_increment || name | varchar(20) | NO | | NULL | || sex | enum('male','female') | NO | | male | || age | int(3) unsigned | NO | | 28 | || hire_date | date | NO | | NULL | || post | varchar(50) | YES | | NULL | || post_comment | varchar(100) | YES | | NULL | || salary | double(15,2) | YES | | NULL | || office | int(11) | YES | | NULL | || depart_id | int(11) | YES | | NULL | |+--------------+-----------------------+------+-----+---------+----------------+#插入记录#三个部门：教学，销售，运营insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部('alex','male',78,'20150302','teacher',1000000.31,401,1),('wupeiqi','male',81,'20130305','teacher',8300,401,1),('yuanhao','male',73,'20140701','teacher',3500,401,1),('liwenzhou','male',28,'20121101','teacher',2100,401,1),('jingliyang','female',18,'20110211','teacher',9000,401,1),('jinxin','male',18,'19000301','teacher',30000,401,1),('成龙','male',48,'20101111','teacher',10000,401,1),('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门('丫丫','female',38,'20101101','sale',2000.35,402,2),('丁丁','female',18,'20110312','sale',1000.37,402,2),('星星','female',18,'20160513','sale',3000.29,402,2),('格格','female',28,'20170127','sale',4000.33,402,2),('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门('程咬金','male',18,'19970312','operation',20000,403,3),('程咬银','female',18,'20130311','operation',19000,403,3),('程咬铜','male',18,'20150411','operation',18000,403,3),('程咬铁','female',18,'20140512','operation',17000,403,3);#ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbkSELECT *FROM emp AS t1INNER JOIN ( SELECT post, max(hire_date) max_date FROM emp GROUP BY post) AS t2 ON t1.post = t2.postWHERE t1.hire_date = t2.max_date;mysql> select (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;+---------------------------------------------------------------------------------------+| (select t2.name from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |+---------------------------------------------------------------------------------------+| 张野 || 格格 || alex || egon |+---------------------------------------------------------------------------------------+rows in set (0.00 sec)mysql> select (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post;+-------------------------------------------------------------------------------------+| (select t2.id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) |+-------------------------------------------------------------------------------------+| 14 || 13 || 2 || 1 |+-------------------------------------------------------------------------------------+rows in set (0.00 sec)#正确答案mysql> select t3.name,t3.post,t3.hire_date from emp as t3 where id in (select (select id from emp as t2 where t2.post=t1.post order by hire_date desc limit 1) from emp as t1 group by post);+--------+-----------------------------------------+------------+| name | post | hire_date |+--------+-----------------------------------------+------------+| egon | 老男孩驻沙河办事处外交大使 | 2017-03-01 || alex | teacher | 2015-03-02 || 格格 | sale | 2017-01-27 || 张野 | operation | 2016-03-11 |+--------+-----------------------------------------+------------+rows in set (0.00 sec)答案一为正确答案，答案二中的limit 1有问题（每个部门可能有>1个为同一时间入职的新员工），我只是想用该例子来说明可以在select后使用子查询可以基于上述方法解决：比如某网站在全国各个市都有站点，每个站点一条数据，想取每个省下最新的那一条市的网站质量信息五 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 ('1', '三年二班'), ('2', '三年三班'), ('3', '一年二班'), ('4', '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 ('1', '生物', '1'), ('2', '物理', '2'), ('3', '体育', '3'), ('4', '美术', '2');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 ('1', '1', '1', '10'), ('2', '1', '2', '9'), ('5', '1', '4', '66'), ('6', '2', '1', '8'), ('8', '2', '3', '68'), ('9', '2', '4', '99'), ('10', '3', '1', '77'), ('11', '3', '2', '66'), ('12', '3', '3', '87'), ('13', '3', '4', '99'), ('14', '4', '1', '79'), ('15', '4', '2', '11'), ('16', '4', '3', '67'), ('17', '4', '4', '100'), ('18', '5', '1', '79'), ('19', '5', '2', '11'), ('20', '5', '3', '67'), ('21', '5', '4', '100'), ('22', '6', '1', '9'), ('23', '6', '2', '100'), ('24', '6', '3', '67'), ('25', '6', '4', '100'), ('26', '7', '1', '9'), ('27', '7', '2', '100'), ('28', '7', '3', '67'), ('29', '7', '4', '88'), ('30', '8', '1', '9'), ('31', '8', '2', '100'), ('32', '8', '3', '67'), ('33', '8', '4', '88'), ('34', '9', '1', '91'), ('35', '9', '2', '88'), ('36', '9', '3', '67'), ('37', '9', '4', '22'), ('38', '10', '1', '90'), ('39', '10', '2', '77'), ('40', '10', '3', '43'), ('41', '10', '4', '87'), ('42', '11', '1', '90'), ('43', '11', '2', '77'), ('44', '11', '3', '43'), ('45', '11', '4', '87'), ('46', '12', '1', '90'), ('47', '12', '2', '77'), ('48', '12', '3', '43'), ('49', '12', '4', '87'), ('52', '13', '3', '87');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 ('1', '男', '1', '理解'), ('2', '女', '1', '钢蛋'), ('3', '男', '1', '张三'), ('4', '男', '1', '张一'), ('5', '女', '1', '张二'), ('6', '男', '1', '张四'), ('7', '女', '2', '铁锤'), ('8', '男', '2', '李三'), ('9', '男', '2', '李一'), ('10', '女', '2', '李二'), ('11', '男', '2', '李四'), ('12', '女', '3', '如花'), ('13', '男', '3', '刘三'), ('14', '男', '3', '刘一'), ('15', '女', '3', '刘二'), ('16', '男', '3', '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 ('1', '张磊老师'), ('2', '李平老师'), ('3', '刘海燕老师'), ('4', '朱云海老师'), ('5', '李杰老师');COMMIT;SET FOREIGN_KEY_CHECKS = 1;复制代码从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表结构为！！！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11 、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 
答案：
#1、查询所有的课程的名称以及对应的任课老师姓名SELECT course.cname, teacher.tnameFROM courseINNER JOIN teacher ON course.teacher_id = teacher.tid;#2、查询学生表中男女生各有多少人SELECT gender 性别, count(1) 人数FROM studentGROUP BY gender;#3、查询物理成绩等于100的学生的姓名SELECT student.snameFROM studentWHERE sid IN ( SELECT student_id FROM score INNER JOIN course ON score.course_id = course.cid WHERE course.cname = '物理' AND score.num = 100 );#4、查询平均成绩大于八十分的同学的姓名和平均成绩SELECT student.sname, t1.avg_numFROM studentINNER JOIN ( SELECT student_id, avg(num) AS avg_num FROM score GROUP BY student_id HAVING avg(num) > 80) AS t1 ON student.sid = t1.student_id;#5、查询所有学生的学号，姓名，选课数，总成绩(注意：对于那些没有选修任何课程的学生也算在内)SELECT student.sid, student.sname, t1.course_num, t1.total_numFROM studentLEFT JOIN ( SELECT student_id, COUNT(course_id) course_num, sum(num) total_num FROM score GROUP BY student_id) AS t1 ON student.sid = t1.student_id;#6、 查询姓李老师的个数SELECT count(tid)FROM teacherWHERE tname LIKE '李%';#7、 查询没有报李平老师课的学生姓名(找出报名李平老师课程的学生，然后取反就可以)SELECT student.snameFROM studentWHERE sid NOT IN ( SELECT DISTINCT student_id FROM score WHERE course_id IN ( SELECT course.cid FROM course INNER JOIN teacher ON course.teacher_id = teacher.tid WHERE teacher.tname = '李平老师' ) );#8、 查询物理课程比生物课程高的学生的学号(分别得到物理成绩表与生物成绩表，然后连表即可)SELECT t1.student_idFROM ( SELECT student_id, num FROM score WHERE course_id = ( SELECT cid FROM course WHERE cname = '物理' ) ) AS t1INNER JOIN ( SELECT student_id, num FROM score WHERE course_id = ( SELECT cid FROM course WHERE cname = '生物' )) AS t2 ON t1.student_id = t2.student_idWHERE 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(没有同时选修指的是选修了一门的，思路是得到物理+体育课程的学生信息表，然后基于学生分组，统计count(课程)=1)SELECT student.snameFROM studentWHERE sid IN ( SELECT student_id FROM score WHERE course_id IN ( SELECT cid FROM course WHERE cname = '物理' OR cname = '体育' ) GROUP BY student_id HAVING COUNT(course_id) = 1 );#10、查询挂科超过两门(包括两门)的学生姓名和班级(求出<60的表，然后对学生进行分组，统计课程数目>=2)SELECT student.sname, class.captionFROM studentINNER JOIN ( SELECT student_id FROM score WHERE num < 60 GROUP BY student_id HAVING count(course_id) >= 2) AS t1INNER JOIN class ON student.sid = t1.student_idAND student.class_id = class.cid;#11、查询选修了所有课程的学生姓名(先从course表统计课程的总数，然后基于score表按照student_id分组，统计课程数据等于课程总数即可)SELECT student.snameFROM studentWHERE sid IN ( SELECT student_id FROM score GROUP BY student_id HAVING COUNT(course_id) = (SELECT count(cid) FROM course) );#12、查询李平老师教的课程的所有成绩记录SELECT *FROM scoreWHERE course_id IN ( SELECT cid FROM course INNER JOIN teacher ON course.teacher_id = teacher.tid WHERE teacher.tname = '李平老师' );#13、查询全部学生都选修了的课程号和课程名(取所有学生数，然后基于score表的课程分组，找出count(student_id)等于学生数即可)SELECT cid, cnameFROM courseWHERE cid IN ( SELECT course_id FROM score GROUP BY course_id HAVING COUNT(student_id) = ( SELECT COUNT(sid) FROM student ) );#14、查询每门课程被选修的次数SELECT course_id, COUNT(student_id)FROM scoreGROUP BY course_id;#15、查询之选修了一门课程的学生姓名和学号SELECT sid, snameFROM studentWHERE sid IN ( SELECT student_id FROM score GROUP BY student_id HAVING COUNT(course_id) = 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）SELECT DISTINCT numFROM scoreORDER BY num DESC;#17、查询平均成绩大于85的学生姓名和平均成绩SELECT sname, t1.avg_numFROM studentINNER JOIN ( SELECT student_id, avg(num) avg_num FROM score GROUP BY student_id HAVING AVG(num) > 85) t1 ON student.sid = t1.student_id;#18、查询生物成绩不及格的学生姓名和对应生物分数SELECT sname 姓名, num 生物成绩FROM scoreLEFT JOIN course ON score.course_id = course.cidLEFT JOIN student ON score.student_id = student.sidWHERE course.cname = '生物'AND score.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 snameFROM studentWHERE sid = ( SELECT student_id FROM score WHERE course_id IN ( SELECT course.cid FROM course INNER JOIN teacher ON course.teacher_id = teacher.tid WHERE teacher.tname = '李平老师' ) GROUP BY student_id ORDER BY AVG(num) DESC LIMIT 1 );#20、查询每门课程成绩最好的前两名学生姓名#查看每门课程按照分数排序的信息，为下列查找正确与否提供依据SELECT *FROM scoreORDER BY course_id, num DESC;#表1：求出每门课程的课程course_id,与最高分数first_numSELECT course_id, max(num) first_numFROM scoreGROUP BY course_id;#表2：去掉最高分，再按照课程分组，取得的最高分，就是第二高的分数second_numSELECT score.course_id, max(num) second_numFROM scoreINNER JOIN ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id) AS t ON score.course_id = t.course_idWHERE score.num < t.first_numGROUP BY course_id;#将表1和表2联合到一起，得到一张表t3，包含课程course_id与该们课程的first_num与second_numSELECT t1.course_id, t1.first_num, t2.second_numFROM ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t1INNER JOIN ( SELECT score.course_id, max(num) second_num FROM score INNER JOIN ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t ON score.course_id = t.course_id WHERE score.num < t.first_num GROUP BY course_id) AS t2 ON t1.course_id = t2.course_id;#查询前两名的学生（有可能出现并列第一或者并列第二的情况）SELECT score.student_id, t3.course_id, t3.first_num, t3.second_numFROM scoreINNER JOIN ( SELECT t1.course_id, t1.first_num, t2.second_num FROM ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t1 INNER JOIN ( SELECT score.course_id, max(num) second_num FROM score INNER JOIN ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t ON score.course_id = t.course_id WHERE score.num < t.first_num GROUP BY course_id ) AS t2 ON t1.course_id = t2.course_id) AS t3 ON score.course_id = t3.course_idWHERE score.num >= t3.second_numAND score.num <= t3.first_num;#排序后可以看的明显点SELECT score.student_id, t3.course_id, t3.first_num, t3.second_numFROM scoreINNER JOIN ( SELECT t1.course_id, t1.first_num, t2.second_num FROM ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t1 INNER JOIN ( SELECT score.course_id, max(num) second_num FROM score INNER JOIN ( SELECT course_id, max(num) first_num FROM score GROUP BY course_id ) AS t ON score.course_id = t.course_id WHERE score.num < t.first_num GROUP BY course_id ) AS t2 ON t1.course_id = t2.course_id) AS t3 ON score.course_id = t3.course_idWHERE score.num >= t3.second_numAND score.num <= t3.first_numORDER BY course_id;#可以用以下命令验证上述查询的正确性SELECT *FROM scoreORDER BY course_id, num DESC;-- 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