一、多表连接查询

　　按照如下命令创建department，employee两个表格：

#创建表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);

#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：不使用任何匹配条件，生成笛卡尔积

　　通过命令select * from employee,department;可以得到的结果如下，结果成为笛卡尔的积。

2、内连接：使用匹配条件，显示匹配的数据行

　　实际上笛卡尔的积是没有意义的，先生成笛卡尔积，然后按照约束条件进行筛选才是我们需要的：

　上述这种利用筛选条件，从笛卡尔的积中选出两表共有的部分，也可以通过如下方式进行实现，称为内连接。

3、左连接：在内连接的基础上增加左边表有右边没有的内容

　　结果如下所示，即使department表没有204这个部门，左表的204部门的员工信息也会显示：

4、右连接：在内连接的基础上增加右边表有左边没有的内容

　　结果如下所示，即使employee表没有员工属于203部门，右表的203部门的信息也会显示：

5、全连接：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

　　在mysql中实际是不支持full join全连接的，但是可以通过union将左、右连接进行连接后去重得到结果，如下：

二、符合条件的连接查询

#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;select employee.name,employee.age from employee inner join department on employee.dep_id=department.id and employee.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;

三、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、带关键字in的查询

#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、带比较运算符的子运算

#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 not 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)

 四、练习

1、创建表

　　按照如下顺序在终端中执行命令，其中init.sql文件见附件。

mysql> create database db1;mysql> use db1;mysql> source ‘init.sql文件路径’

/* 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;

2、题目及答案

#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.tname from    course inner 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,count(sid) from student group by gender;#3、查询物理成绩等于100的学生的姓名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 student.sname,t.avg_num from student inner join(    select student_id,avg(num) avg_num from score         group by student_id        having avg(num)>80) t     on student.sid=t.student_id;#5、查询所有学生的学号，姓名，选课数，总成绩select student.sid,student.sname,t.course_num,t.total_score fromstudent inner join(    select student_id,count(sid) course_num,sum(num) total_score from score         group by student_id) t on student.sid=t.student_id;#6、 查询姓李老师的个数select count(tid),group_concat(tname) 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')));#8、 查询物理课程比生物课程高的学生的学号select student_id from    (select t1.student_id,t1.num biology,t2.num physics from        (select student_id,course_id,num from score where course_id = (            select cid from course where cname ='生物')) t1    inner join         (select student_id,course_id,num from score where course_id = (            select cid from course where cname ='物理')) t2     on t1.student_id=t2.student_id) t3    where physics>biology;#9、 查询没有同时选修物理课程和体育课程的学生姓名select t1.sid from     (select sid from student where sid not in         (select student_id from score where course_id =            (select cid from course where cname='物理'))) t1     inner join     (select sid from student where sid not in         (select student_id from score where course_id =            (select cid from course where cname='体育'))) t2     on t1.sid=t2.sid;     #10、查询挂科超过两门(包括两门)的学生姓名和班级select sname,caption from     (select class_id,sname from student where sid in         (select student_id from             (select sid,student_id from score where num<60) t1            group by student_id            having count(sid)>=2))t2inner join class t3 on t2.class_id=t3.cid;#11、查询选修了所有课程的学生姓名select sname from student where sid in     (select student_id from score        group by student_id        having count(sid)=(select count(cid) course_count from course));#12、查询李平老师教的课程的所有成绩记录select cname,num from     (select cid,cname from course where teacher_id=        (select tid from teacher where tname='李平老师')) t1inner join score t2 on t1.cid=t2.course_id;     #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) student_count from student));#14、查询每门课程被选修的次数select cname,select_count from     (select course_id,count(sid) select_count from score        group by course_id) t1inner join course t2 on t1.course_id=t2.cid;#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）select num from score group by num 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 sname,avg_num from     (select student_id,avg(num) avg_num from score         group by student_id        having avg(num)>85) t1inner join student t2 on t1.student_id=t2.sid;#18、查询生物成绩不及格的学生姓名和对应生物分数select sname,num from     (select student_id,num from score where course_id=        (select cid from course where cname='生物')        and num<60) t1inner join student t2  on t1.student_id=t2.sid;#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 sname from student where sid 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')        )        group by student_id        having avg(num)=            (select avg(num) from score where course_id in                 (select cid from course where teacher_id=                    (select tid from teacher where 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#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