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Primary Keys and Auto Increment Fields
This is true for al most all of the relational database management systems that each table of the database has to have a primary key field. A primary key field is used to uniquely identify the rows or records in a table. Each primary key value must be unique within the table. Furthermore, the primary key field cannot be null because the database engine requires a value to locate the record.

Often the primary key field is some sort of a number e.g. ID, and is often used with the AUTO_INCREMENT setting. When we use AUTO_INCREMENT with primary key, or any other field, it means that the value of the field is to be incremented automatically, usually by 1 each time a new record is added.

To ensure that the primary key field is not to be null, we must add the NOT NULL setting to the field.

Example:
The following SQL code sets the studentID field as the primary key field.

$sql = "CREATE TABLE Students
      (
      studentID int NOT NULL AUTO_INCREMENT,
      PRIMARY KEY(studentID),
      FirstName varchar(15),
      LastName varchar(15),      
	  Age int
      )";
      mysql_query($sql,$con); 



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