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+### 笔记
+
+表之间的关系：
+
+1.一对一的关系：将其中任意一个表的主键放在另一个表中当外键
+
+2.一对多的关系：将一的主键放在多的表中当外键
+
+3.多对多的关系：必须第三张表，将前面两张表的主键放在第三张表当外键
+
+ER图：实体关系图 
+
+### 作业
+
+```mysql
+create DATABASE XUE charset utf8;
+
+use XUE;
+
+create table yuanxi(
+    yuan_id int PRIMARY key,
+    yuan_name varchar(50) not null
+);
+
+insert into yuanxi values
+(1,'软件工程学院');
+
+
+create table zhuanye(
+    zhuan_id int PRIMARY key,
+    zhuan_name varchar(50) not null,
+    yuan_id int not null,
+    FOREIGN key(yuan_id) REFERENCES yuanxi(yuan_id)
+);
+
+insert into zhuanye values
+(1,'软件技术',1);
+
+create table class(
+    class_id int PRIMARY key,
+    class_name varchar(50) not null,
+    zhuan_id int not null,
+    FOREIGN key(zhuan_id) REFERENCES zhuanye(zhuan_id)
+);
+
+insert into class values
+(1,'前端',1),
+(2,'后端',1),
+(3,'新媒体',1);
+
+create table student(
+    stu_id int PRIMARY key,
+    stu_name varchar(50) not null,
+    class_id int not null,
+    age int,
+    address varchar(50),
+    FOREIGN key(class_id) REFERENCES class(class_id)
+);
+
+insert into student values
+(1,'小明',1,18,'翻斗花园'),
+(2,'小红',2,16,'狼堡'),
+(3,'小花',3,12,'羊村');
+
+create table course(
+    cou_id int PRIMARY key,
+    cou_name varchar(50) not null
+);  
+
+insert into course values
+(001,'JAVA'),
+(002,'MySQL'),
+(003,'Html');
+
+CREATE TABLE teacher(
+	teacher_id int  PRIMARY KEY,                        
+	teacher_name VARCHAR(10) not null,                          
+	cou_id int not null,                                     
+	FOREIGN KEY (course_id) REFERENCES course(cou_id)
+	);
+INSERT INTO teacher VALUES
+(1,'张三',001),
+(2,'李四',002),
+(3,'王五',003);
+
+CREATE TABLE xuanxiu( 
+	cou_id int NOT NULL PRIMARY KEY,                       
+	cou_name VARCHAR(50) not null,                         
+	teacher_id int not null                                      
+	);
+INSERT INTO xuanxiu VALUES
+(001,'JAVA',3),
+(003,'Html',2),
+(002,'MySQL',1);
+```
+