Design of database table foreign key

Database table foreign key design

One to many ：

	 One to many , Two tables , More tables with foreign keys ！！！！！！！！！！！！

t_class  Class table 
		classno(pk)			classname
		------------------------------------------------------
		100					 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
		101					 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 

t_student  Student list 
no(pk)			name				cno(FK quote t_class This watch is classno)
----------------------------------------------------------------
1					jack				100
2					lucy				100
3					lilei				100
4					hanmeimei		100
5					zhangsan			101
6					lisi				101
7					wangwu			101
8					zhaoliu			101

Many to many ：

	 Many to many , Three tables , Relationship table two foreign keys ！！！！！！！！！！！！！！！

In order to make the above table meet the second paradigm , Need this design ：
Use 3 A watch To express Many to many The relationship between ！！！！

 Student list 
 Student number (pk)		 Student name 
------------------------------------
1001					 Zhang San 
1002					 Li Si 
1003					 Wang Wu 

 Teachers list 
 Teacher number (pk)		 Teacher's name 
--------------------------------------
001					 Teacher wang 
002					 Miss Zhao 

 Student teacher relationship form 
id(pk)			 Student number (fk)			 Teacher number (fk)
------------------------------------------------------
1						1001						001
2						1002						002
3						1003						001
4						1001						002

one-on-one ：

Don't you just put one-on-one in a table ？ Why split the table ？
In actual development , There may be too many fields in a table , Too big . Split the table at this time .
How to design one-on-one ？
Before splitting the table ： A watch

t_user
id		login_name		login_pwd		real_name		email				address........
---------------------------------------------------------------------------
1			zhangsan		123				 Zhang San 				[email protected]
2			lisi			123				 Li Si 				[email protected]

This huge table is recommended to be split into two ：

t_login  Login information form 
id(pk)		login_name		login_pwd	
---------------------------------
1				zhangsan		123			
2				lisi			123	

t_user  User details table 
id(pk)		real_name		email				address........	login_id(fk+unique)
-----------------------------------------------------------------------------------------
100			 Zhang San 				[email protected]						1
200			 Li Si 				[email protected]							2

 formula ： one-on-one , The foreign key is unique ！！！！！！！！！！

Foreign key constraints （foreign key, abbreviation FK）

 Terms related to foreign key constraints ：
	 Foreign key constraints ： A constraint （foreign key）
	 Foreign key field ： A foreign key constraint is added to this field 
	 Foreign key value ： Every value in the foreign key field .


 Business background ：
	 Please design the database table , To describe “ Class and students ” Information about ？

First option ： Classes and students are stored in a table ？？？

t_student
		no(pk)			name		classno			classname
		----------------------------------------------------------------------------------
		1					jack			100			 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
		2					lucy			100			 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
		3					lilei			100			 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
		4					hanmeimei	100			 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
		5					zhangsan		101			 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 2 class 
		6					lisi			101			 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 2 class 
		7					wangwu		101			 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 2 class 
		8					zhaoliu		101			 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 2 class 

Analyze the disadvantages of the above scheme ：
data redundancy , Space waste ！！
This design is a failure ！

Second option ： Class a table 、 Students a table

t_class  Class table 
		classno(pk)			classname
		------------------------------------------------------
		100					 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 
		101					 Senior 3, No. 2 middle school, Yizhuang Town, Daxing District, Beijing 1 class 

t_student  Student list 
no(pk)			name				cno(FK quote t_class This watch is classno)
----------------------------------------------------------------
1					jack				100
2					lucy				100
3					lilei				100
4					hanmeimei		100
5					zhangsan			101
6					lisi				101
7					wangwu			101
8					zhaoliu			101

	 When cno When the field has no constraints , May cause invalid data . There could be a 102, however 102 Class does not exist .
	 So to make sure cno The values in the field are 100 and 101, Need to give cno Field add foreign key constraint .
	 that ：cno Fields are foreign key fields .cno Each value in the field is a foreign key value .

 Be careful ：
			t_class It's the father's watch 
			t_student It's a subtable 

			 The order in which tables are deleted ？
				 Delete the sub first , Then delete the father .

			 The order in which tables are created ？
				 Create the parent first , Create Jianzi again .

			 The order in which data is deleted ？
				 Delete the sub first , Then delete the father .

			 The order in which the data is inserted ？
				 Insert parent first , Then insert the sub .

Be careful ：

• The foreign key in the child table refers to a field in the parent table , Must the referenced field be a primary key ？ Not necessarily the primary key , But at least it has unique constraint .
• The foreign key value can be NULL.