SQL PRIMARY KEY instance tutorial

SQL PRIMARY KEY instance tutorial

Database primary key constraints
The primary key constraint uniquely identifies each record in a database table.

The primary key must contain unique values.

The primary key column cannot contain null values.

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


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Create Table with database primary key constraints
When the following SQL statement creates the "P_Id" column on a primary key, "person" creates a table:

MySQL:

Create table Persons(P_Id int not null,LastName varchar (255) not null,FirstName varchar (255), Address varchar (255), City varchar (255), primary key (P_Id ))

Change table with database primary key constraints
To create a column named "P_Id" for the primary key constraint, the table has been created. Use the following SQL statement:
 
Alter table PersonsADD CONSTRAINT pk_PersonID primary key (P_Id, LastName)
 
Look at mysql
MySQL:

  
  


   
   
 
    
 
     
Alter table PersonsDROP PRIMARY KEY