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+## 笔记
+
+```sql
+关联查询：两个或更多个表一起查询。
+又称为联合查询，多表查询。
+
+1、关联查询的结果一共有7种：
+两个表的记录分为四种：
+①A表中的记录能在B表中找到对应的记录
+②A表中的记录在B表中找不到对应的记录
+③B表中的记录可以在A表找到对应的记录
+④B表中的记录在A表中找不到对应的记录
+
+（1）A∩B      ①③
+（2）A        ①②
+（3）A-A∩B    ②
+（4）B        ③④
+（5）B-A∩B    ④
+（6）A∪B      ①②③④
+（7）A∪B-A∩B  ②④
+
+2、两个表要一起查询，要有前提条件：有关联
+就是有相同逻辑意义和数据类型的字段。
+
+3、如何实现7种查询结果
+（1）内连接  inner join
+（2）外连接  outer join 
+左外连接   left outer join 或 left join
+右外连接   right outer join 或 right join
+全外连接   full outer join 或 full join  -- union 代替
+
+但是，mysql不支持全外连接，没有full join。
+mysql使用union关键字合并其他的查询结果实现全外连接的效果。
+
+内连接 ==> A∩B
+左连接 ==> A 或 A-A∩B
+右连接 ==> B 或 B-A∩B
+全外连接 ==> A∪B 或 A∪B - A∩B
+     左连接的A union 右连接的B    得到  A∪B
+     左连接的A-A∩B  union 右连接B-A∩B 得到 A∪B - A∩B
+
+4、内连接 inner join 
+A表 inner join B表 on 关联条件
+*/
+
+```
+
+## 作业
+
+```sql
+CREATE database ku charset utf8;
+USE ku;
+CREATE table Student(
+Sno    varchar (20) primary key,
+Sname    varchar (20) NOT NULL,
+Ssex    varchar (20) NOT NULL,
+Sbirthday    datetime,
+Class    varchar (20)
+);  
+DROP TABLE Student;
+#foreign key 
+#foreign key （从表的某个字段) references 主表名(被参考字段)
+CREATE table Course(
+Cno    varchar (20) PRIMARY key,
+Cname    varchar (20) not null,
+Tno    varchar (20) not null,
+foreign key (Tno) references Teacher(Tno)
+);
+drop table Course;
+CREATE table Score(
+Sno    varchar (20),
+Cno    varchar (20),
+Degree    Decimal(4,1),
+PRIMARY key(Sno,Cno),
+foreign key (Sno) REFERENCES Student(Sno),
+foreign key (Cno) REFERENCES Course(Cno)
+);
+ drop table Score;
+CREATE table Teacher(
+Tno    varchar (20) PRIMARY key,
+Tname    varchar (20),
+Tsex    varchar (20),
+Tbirthday    datetime,
+Prof    varchar (20),
+Depart    varchar (20)
+);
+drop TABLE Teacher;
+```
+
+```sql
+insert into Student VALUES ('108','曾华','男',    '1977-9-1','95033'),
+('105','匡明','男','1975-10-2','95031'),
+('107','王丽','女','1976-1-23','95033'),
+('101','李军','男','1976-2-20','95033'),
+('109','王芳','女','1975-2-10','95031'),
+('103','陆君','男','1974-6-3','95031');
+
+insert into Course VALUES ('3-105','计算机导论','825'),
+('3-245','操作系统','804'),
+('6-166','数字电路','856'),
+('9-888','高等数学','831');
+insert into Score values 
+    ('103','3-245',86),
+    ('105','3-245',75),
+    ('109','3-245',68),
+    ('103','3-105',92),
+    ('105','3-105',88),
+    ('109','3-105',76),
+    ('101','3-105',64),
+    ('107','3-105',91),
+    ('108','3-105',78),
+    ('101','6-166',85),
+    ('107','6-166',79),
+    ('108','6-166',81);
+insert into Teacher values 
+        ('804','李诚','男','1958-12-2','副教授','计算机系'),
+        ('856','张旭','男','1969-3-12','讲师','电子工程系'),
+        ('825','王萍','女','1972-5-5','助教','计算机系'),
+        ('831','刘冰','女','1977-8-14','助教','电子工程系');
+
+
+```
+
+```sql
+#①　查询Score表中的最高分的学生学号和课程号
+SELECT max(Degree) from Score;
+#②　查询所有学生的Sname、Cno和Degree列。
+select Sname,Cno,Degree from student inner join score on score.Sno=student.Sno;
+#③　查询所有学生的Sno、Cname和Degree列。
+select Sno,Cname,Degree from Score inner join Course  on Course.Cno=Score.Cno;
+#④　查询所有学生的Sname、Cname和Degree列。
+SELECT Sname,Cname,Degree from Score right join Course on Score.Cno=Course.Cno right join Student on Score.Sno=Student.Sno;
+#⑤　查询“95033”班学生的平均分。
+select avg(Degree) from Student inner join Score on Score.Sno=Student.Sno where Class='95033';
+#⑥　查询选修“3-105”课程的成绩高于“109”号同学成绩的所有同学的记录。
+select Degree,Cno,Student.Sno from Student inner join Score on Score.Sno=Student.Sno where Cno='3-105' and Degree>72;
+#⑦　查询score中选学多门课程的同学中分数为非最高分成绩的记录。
+SELECT Cname,Degree from Course left join Score on Score.Cno=Course.Cno where Degree<(select max(Degree) from Score);
+#⑧　查询成绩高于学号为“109”、课程号为“3-105”的成绩的所有记录。
+select Sno,Degree from Score where Degree>76;
+#⑨　查询和学号为108的同学同年出生的所有学生的Sno、Sname和Sbirthday列。
+select Sno,Sname,Sbirthday from Student where Sno='108';
+#⑩　查询“张旭“教师任课的学生成绩。
+SELECT Tname,Degree,Cname from Course inner join Score on Course.Cno=Score.Cno right join Teacher on Teacher.Tno=Course.Tno where Tname='张旭';
+
+#⑪　查询选修某课程的同学人数多于5人的教师姓名。
+select Tname from Course inner join Score on Course.Cno=Score.Cno inner join Teacher on Teacher.Tno=Course.Tno having count(Score.Cno)>5;
+#⑫　查询出“计算机系“教师所教课程的成绩表。
+select Tname,Degree from Course inner join Score on Course.Cno=Score.Cno right join Teacher on Teacher.Tno=Course.Tno where Tname='李诚' or Tname='王萍';
+#⑬　查询“计算机系”与“电子工程系“不同职称的教师的Tname和Prof。
+select Tname,Prof,Depart from Teacher ;
+#⑭　查询选修编号为“3-105“课程且成绩至少高于选修编号为“3-245”的同学的Cno、Sno和Degree,并按Degree从高到低次序排序。
+select Degree,Cno,Student.Sno from Student inner join Score on Score.Sno=Student.Sno where Cno='3-105' and Degree>72 order by Degree desc;
+#⑮　查询选修编号为“3-105”且成绩高于选修编号为“3-245”课程的同学的Cno、Sno和Degree.
+select Degree,Cno,Student.Sno from Student inner join Score on Score.Sno=Student.Sno where Cno='3-105' and Degree>72;
+#⑯　查询成绩比该课程平均成绩低的同学的成绩表。
+select Sname,Degree from Student right join Score on Student.Sno=Score.Sno where Degree<(select avg(Degree) from Score);
+#查询所有任课教师的Tname和Depart.
+select Tname,Depart from Course inner join Score on Course.Cno=Score.Cno right join Teacher on Teacher.Tno=Course.Tno where Degree is not null;
+#查询所有未讲课的教师的Tname和Depart.
+select Tname,Depart from Course inner join Score on Course.Cno=Score.Cno right join Teacher on Teacher.Tno=Course.Tno where Degree is null;
+#查询“男”教师及其所上的课程。
+select Tname,Tsex,Cname from Teacher inner join Course on Course.Tno=Course.Tno where Tsex='男';
+#查询最高分同学的Sno、Cno和Degree列。
+select Sname,Degree,Cno,Student.Sno from Student inner join Score on Student.Sno=Score.Sno where Degree=(select max(Degree) from Score);
+#查询和“李军”同性别的所有同学的Sname.
+select Sname,Ssex from Student where Ssex='男';
+#查询和“李军”同性别并同班的同学Sname.
+select Sname,Class from Student where Ssex='男' and class='95033';
+#查询所有选修“计算机导论”课程的“男”同学的成绩表。
+select Cname,Ssex from Course inner join Score on Course.Cno=Score.Cno inner join Student on Student.Sno=Score.Sno where Cname='计算机导论' and Ssex='男';
+
+
+```