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.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 |+------+------------+--------+------+--------+------+--------------+
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：= 、 !=、> 、
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，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)
五 综合练习
init.sql文件内容
View Code 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
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
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 gender3、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 JOINcourse 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 scON s.sid=sc.student_id5、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_id6、 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 t1JOIN(SELECT student_id,num FROM score s INNER JOIN course c ON c.cid=s.course_id WHERE c.cname="生物")AS t2 ONt1.student_id=t2.student_id WHERE t1.num>t2.num9、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.cidSELECT 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.cidSELECT 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_id11 、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_id15、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 DESC17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_idLEFT JOIN course c ON s.course_id=c.cid WHERE c.cname="生物" AND s.num<6019、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.sidLEFT JOIN course c ON c.cid=s.course_id WHERE c.cid IN (SELECT c.cid FROM course c LEFT JOINteacher 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_idLEFT 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 2SELECT stu.sname FROM student stu LEFT JOIN score s ON s.student_id =stu.sidLEFT 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