Day13 Mysql multi-table relationship

One, foreign key

• Now we have two tables, "Classification Table" and "Commodity Table". In order to indicate which category the product belongs to, we usually add a column to the product table to store the classified cid information. This column is called: external Key .
  
  
• At this time, the
  "category category" is called the primary table, and the cid is called the primary key.
  "Products table products" is called: from the table, category_id is called a foreign key.
  We describe the primary and foreign key relationship through the primary key of the primary table and the foreign key of the secondary table, which presents a one-to-many relationship.
• Foreign key characteristics:
  The value of the foreign key from the table is a reference to the primary key of the main table.
  The type of the foreign key from the table must be consistent with the primary key type of the primary table
• Declare foreign key constraints

Syntax: alter table from the table add [constraint] [foreign key name] foreign key (from the table foreign key field name) references the main table (the primary key of the primary table);
where [foreign key constraint] is used to delete foreign key constraints, generally It is recommended that "_fk" end
alter table from the table drop foregin key foreign key name;

• Purpose of using foreign keys: to
  ensure data integrity

Two, the relationship between the table and the table

The relationship between the table and the table is the relationship between the table and the table data.

1. One-to-one relationship: (understand)

• There are not many applications in actual development, because one to one can create a table.

2. One-to-many relationship:

• Common examples: customers and orders, categories and commodities, departments and employees.
• The principle of one-to-many table creation: create a field in the slave table (many side), and the field is used as a foreign key to point to the primary key of the primary table (one side).
  

3. Many-to-many relationship:

• Common examples: students and courses, products and orders, people and roles.

• The principle of many-to-many relationship table creation : You need to create a third table with at least two fields in the middle table, and these two fields are used as foreign keys to point to the primary key of each . (That is to split one many-to-many into two one-to-many)
  

• Both tables are the master table, and the third table is the slave table, providing two fields, both of which are foreign keys.

Three, create a table to achieve

1. One-to-many: categories and commodities

   #创建数据库
   create database web01;

   #使用数据库
   create web01;

   #创建分类表
   create table category(
      cid int primary key auto_increment, #主表的主键
      cname varchar(20)   #分类名称
   );

   #商品表
   create table products(
      pid int primary key auto_increment,
      name varchar(20),
      price double
   );

   #添加外键字段category_id
   alter table products add category_id int;

   #添加外键约束
   alter table products add foreign key(category_id) references     category(cid);

2. Many to many: orders and commodities

   #订单表
   create table orders(
      oid int primary key auto_increment,
      totalprice double #总计
   );

   #订单项表
   create table orderitem(
      oid int primary key, #订单id
      pid int primary key  #商品id
   );

   #订单表和订单项表的主外键关系
   alter table orderitem add foreign key(oid) references orders(oid);

   #商品表和订单项表的主外键关系
   alter table orderitem add foreign key(pid) references products(pid);

to sum up:

• Add data: master table: add at will; slave table: limited by the master table, you cannot add data that does not exist in the master table.
• Delete data: Main table: If a row of data is dependent on the slave table and has been used in the slave table, it cannot be deleted ; From the table: delete at will.
• If you want to delete the data in the main table that is related to the slave table, you can do this:
  1. Lift the constraint relationship of the master-slave table
  2. First delete the data in the slave table that is related to the main table, and then delete the data in the main table .