A brief discussion on the difference between MYSQL primary key constraint and unique constraint

Primary key constraint

PRIMARY KRY The primary key is unique. A table can only have one primary key.
AUTO_INCREMENT must be used with the primary key. The primary key must be NOT NULL.
Features: cannot be empty, no duplication

##No constraintsCreate table stu0(
Id int 
Name varcahr(50) 
) 
Insert into stu0(name)value("Zhang Sanfeng");
##Method 1: Create a table and add a primary key constraintCreate table stu1(
Id int primary key;
Name varchar(50) 
)
##Method 2:
Create table stu2（
Id int, 
Name varchar(50),
Primary key(name)
)

Success: insert intostu1(in,name)value("2,张三丰"); Success Test 1: insert into stu(id,name)value(null,"张三丰"); #Failed, prompt cannot be empty Test 2:
Insert duplicate value: error
Duplicate entry'2' for key 'PRIMARY'
Select *from stu1;

Unique Constraint

UNIQUE KEY unique constraint can ensure the uniqueness of data. Each data table can have multiple unique constraints.

Unique constraint
No repetition, can be empty

##Add a unique constraint to the name Create table stu3(
 Id int primary key,
 Name varchar(50) unique
)
 Insert into stu3(id,name)value(1,"Zhang Sanfeng");
 
 Insert into stu3(id,name)value(2,"Zhang Sanfeng");
 ERROR 1062(23000):Duplicate entry '张三丰' for key 'name'

Insert into stu3(id,name)value(2,"张三");

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