1 How to distinguish between main table and sub table?
The parent table sets a primary key, and the child table sets a foreign key
When a one-to-many relationship is established between two tables , the "one" end is the parent table, and the "many" end is the child table.
Which class does a student belong to? The student table is the child table and the class table is the main table
Which department does an employee belong to? The employee table is a child table, and the department table is the main table
2How to establish the main external construction constraints
FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
外键 p_id 参照 主表Persons 的主键 （P_Id）3 The main external constraints need to create tables and data changes
-- 创建班级表
CREATE TABLE class(
c_id INT NOT NULL AUTO_INCREMENT,
c_name VARCHAR(22) ,
c_type INT ,
c_desc VARCHAR(122),
PRIMARY KEY(c_id)
);
-- 创建子表
/**
先创建主表，在创建 子表
先删除子表，在删除主表
*/
CREATE TABLE student(
s_id INT NOT NULL AUTO_INCREMENT,
s_name VARCHAR(22),
s_sex CHAR(2),
s_address VARCHAR(122),
c_id INT ,
PRIMARY KEY(s_id),
FOREIGN KEY(c_id) REFERENCES class(c_id)
);
-- 插入数据
/*
在子表插入数据时，保证外键，在主表有数据的存在
*/
INSERT INTO class VALUES (NULL,'3班',2,'xxxxxxxx');
INSERT INTO student VALUES (NULL,'张三','男','北京市，xxxx,',1);
INSERT INTO student VALUES (NULL,'张三2','男','北京市，xxxx,',2);
-- 删除数据
/*
删除数据时，如果是主表，那么在子表中没有关联的数据才可以删除
*/
DELETE FROM class WHERE c_id=1;
DELETE FROM student WHERE s_id =3 ;
-- 更新
/*
更新子表中的外键，要保证主表中有对应的数据
*/
UPDATE class SET c_name='3班' WHERE c_id=3;
UPDATE student SET c_id=4 WHERE s_id=4;
4 other constraints
NOT NULL - 指示某列不能存储 NULL 值。
UNIQUE - 保证某列的每行必须有唯一的值。
PRIMARY KEY - NOT NULL 和 UNIQUE 的结合。确保某列（或两个列多个列的结合）有唯一标识，有助于更容易更快速地找到表中的一个特定的记录。
FOREIGN KEY - 保证一个表中的数据匹配另一个表中的值的参照完整性。(上面讲解过)
CHECK - 保证列中的值符合指定的条件。
DEFAULT - 规定没有给列赋值时的默认值。Details:
http://www.w3school.com.cn/sql/sql_constraints.asp