What are the conditions when MySQL creates a table?

Due to the addition, deletion and modification of records in the emp table, a script is re-created here and used

create database bjpowernnode;
use bjpowernode;
source C:\Users\Administrator\Desktop\bjpowernode.sql;

constraints

1. What are constraints?

• Constraints are the restrictions in the table

• The keyword of the constraint is: constraint

2. Classification of constraints

• Non-null constraint not null

• Unique Sexual constraints unique

• Primary key constraints primary key

• ##Foreign key constraints

  foreign key

• Checking constraints MySQL database does not support it, Oracle database supports it

1. not null (non-null constraint)

The fields with not null constraints cannot be null values. Specific data must be created.

Create a table and add non-null constraints to the fields [The user’s email address cannot be null]

drop table if exists t_user;
create table t_user(
        id int(10),
        name varchar(32) not null,
        email varchar (32)
);

2. unique (unique constraint)

Create a table to ensure that the email address is unique

create table t_user(
id int(10),
name varchar(32) not null,
email varchar(128) unique
);

The fields of unique constraints cannot be repeated, but can be null

The above constraints are column-level constraints

Table-level constraints:

 create table t_user(
     id int(10),
     name varchar(32),
     email varchar(128),
     unique(email)
 );

1. Use table-level constraints to add constraints to multiple fields

 create table t_user(
     id int(10),
     name varchar(32),
     email varchar(128),
     unique(name,email)
);

2. Table-level constraints can give the constraint a name, and later delete the constraint through this name

   create table t_user(
       id int(10),
       name varchar(32),
       email varchar(128),
       constraint t_user_email_unique unique(email)
 );

##not null and unique can be used together

3. primary key (primary key constraint)

1. Terms related to primary key:

Primary key constraint
Primary key field
Primary key value
##2. The relationship between the above three:

After adding a primary key constraint to a field in the table, the field is called the primary key field

• Each occurrence in the primary key field The data are all called primary key values
3. After adding a primary key constraint to a field, the field cannot be repeated or empty

The primary key constraint has the same effect as ''not null unique'', but the essence is different.

• Primary key constraint can also achieve ''not null unique'' In addition to

• the primary key field will also be added with ''index-index'' by default
4. A table should have Primary key field, if not, it means that this table is invalid

The primary key value is the unique identifier of the current row of data

• The primary key value is the ID number of the current row of data

• Even if the two rows of record data in the table are exactly the same,

• But because If the primary key values ​​are different, they are considered to be two completely different fields in the two rows
5. Whether it is a single primary key or a composite primary key, a table can only have one primary key constraint.

Adding a primary key constraint to a field is called a single primary key constraint

• Adding a primary key constraint to multiple fields jointly is called a single primary key constraint It is called a composite primary key
6. Primary keys are classified according to their properties:

Natural primary key: The primary key value is a natural number , this primary key has nothing to do with the current business

• Business primary key: The primary key value is closely related to the current business

• When the business changes , the primary key value will always be affected, so the business primary key is of little use.

• Single primary key, column-level constraints

create table t_user(
    id int(10) primary key,
    name varchar(32)
);

Single primary key, table-level constraints

 create table t_user(
     id int(10),
     name varchar(32),
     primary key(id)
);

Composite primary key: only table-level constraints can be used

mysql> create table t_user(
    -> id int(10),
    -> name varchar(32),
    -> primary key(id,name)
    -> );

auto_increment：主键自增

MySQL数据管理系统中提供了一个自增的数字，专门用来自动生成主键值

主键值不需要用户维护，也不需要用户提供了，自动生成的，

这个自增的数字默认从1开始以1递增：1,2,3,4，....

mysql> create table t_user(
    -> id int(10) primary key auto_increment,
    -> name varchar(32)
    -> );

4. foreign key（外键约束）

1.外键约束涉及到的术语：

• 外键约束

• 外键值

• 外键字段

2.以上三者之间的关系：

• 某个字段添加外键约束以后称为外键字段

• 外键字段中的每一个数据称为外键值

3.外键分为单一外键和复合外键

• 单一外键：给一个字段添加外键约束

• 复合外键：给多个字段添加外键约束

4.一张表中可以有多个外键字段

设计一个数据库表，用来存储学生和班级信息，给出两种解决方案：

学生信息和班级信息之间的关系：一个班级对应多个学生，这是典型的一对多的关系

在多的一方加外键

第一种设计方案：将学生信息和班级信息存储到一张表中

第二种设计方案：将学生信息和班级信息分开两张表存储，学生表+班级表

• 学生表 t_student

sno(主键约束)	sname	classno（外键约束）
1	jack	100
2	lucy	100
3	kk	100
4	smith	200
5	frank	300
6	jhh	300

• 班级表t_calss

cno(主键约束)	cname
100	高三1班
200	高三2班
300	高三3班

为了保证t_student 表中的classno字段中的数据必须来自于t_class表中的cno字段中的数据，有必要给t_student表中的classno字段添加外键约束，classno称为外键字段，该字段中的值称为外键值。

注意：

1.外键值可以为空

2.外键字段必须得引用这张表中的主键吗？

• 外键字段引用一张表的字段的时候，被引用的字段必须具备唯一性

• 即具有unique约束，不一定非是主键

3.班级表为父表，学生表为子表

• 应该先创建父表，再创建子表

• 删除数据时，应该先删除子表中的数据，再删除父表中的数据

• 插入数据时，应该先插入父表中的数据，再删除子表中的数据

DROP TABLE IF EXISTS t_student;
DROP TABLE IF EXISTS t_class;
 
 CREATE TABLE t_class(
 cno INT(3) PRIMARY KEY,
 cname VARCHAR(128) NOT NULL UNIQUE
 );
 
 CREATE TABLE t_student(
 sno INT(3) PRIMARY KEY,
 sname VARCHAR(32) NOT NULL,
 classno INT(3),-- 外键
 CONSTRAINT t_student_class_fk FOREIGN KEY(classno) REFERENCES t_class(cno)
 );
 
 INSERT INTO t_class(cno,cname) VALUES(100,'高三1班');
 INSERT INTO t_class(cno,cname) VALUES(200,'高三2班');
 INSERT INTO t_class(cno,cname) VALUES(300,'高三3班');
 
 INSERT INTO t_student(sno,sname,classno) VALUES(1,'jack',100);
 INSERT INTO t_student(sno,sname,classno) VALUES(2,'lucy',100);
 INSERT INTO t_student(sno,sname,classno) VALUES(3,'hh',100);
 INSERT INTO t_student(sno,sname,classno) VALUES(4,'frank',200); 
 INSERT INTO t_student(sno,sname,classno) VALUES(5,'smith',300);
 INSERT INTO t_student(sno,sname,classno) VALUES(6,'jhh',300);
 
 SELECT * FROM t_student;
 SELECT * FROM t_class;
 
-- 添加失败，因为有外键约束 
 INSERT INTO t_student(sno,sname,classno) VALUES(8,'kk',500);

重点：典型的一对多关系，设计时在多的一方加外键

5. 级联更新与级联删除

在删除父表中的数据的时候，级联删除子表中的数据

在更新父表中的数据的时候，级联更新子表中的数据

以上的级联更新和级联删除谨慎使用，

因为级联操作会使数据数据改变或删除，数据是无价的。

语法：

• 级联更新：on update cascase

• 级联删除：on delete cascase

MySQL中对于有些约束的修改比较麻烦，所以应该先删除约束，再添加约束

删除外键约束：

alter table t_student drop foreign key t_student_class_fk

添加外键约束并级联更新：

alter table t_student add constraint t_student_class_fk foreign key(classno)
references t_class(no) on delete cascade;

添加外键约束并级联删除：

alter table t_student add constraint t_student_class_fk foreign key(classno)
references t_class(no) on update cascade;

级联删除

级联更新

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