SQL PRIMARY KEY Instance Tutorial

SQL PRIMARY KEY Instance Tutorial

Database PRIMARY KEY constraint
The PRIMARY KEY constraint uniquely identifies each record in a database table.

The primary key must contain unique values.

Primary key columns cannot contain null values.

Each table should have a primary key, and each table can have only one primary key.


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Database primary KEY constraint creation table
When the following SQL creates a "p_id" column on a primary key, "person" is creating a table:

MySQL's:

CREATE TABLE Persons
(
p_id int not null,
LastName varchar (255) is not null 
 , FirstName varchar (255), Address
varchar (255), City
varchar (255),
PRIMARY KEY (p_id)
)

 
   
   

    
     Database primary KEY constraint changes table 
    
    
 to create a primary key constraint, the table is already established, using the following sql: 
    
    
 
    
    
 p_id; 
 Pre>alter TABLE Persons ADD CONSTRAINT pk_personid PRIMARY KEY (p_id,lastname) 
   
   
 
   
 
 look at mysql<
   /pre> 
 
  MySQL:  
 
    
    
 
     
     
 
      
 
       
 
 ALTER table P Ersons DROP PRIMARY KEY