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Primary Keys and Auto Increment Fields

Posted By:Shashi Ray       Posted Date: September 28, 2009    Points: 10    Category: SQL Query    URL: http://www.dotnetspark.com  
 

Primary Keys and Auto Increment Fields.

Each table should have a primary key field.

A primary key is used to uniquely identify the rows 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.

The following example sets the personID field as the primary key field. The primary key field is often an ID number, and is often used with the AUTO_INCREMENT setting. AUTO_INCREMENT automatically increases the value of the field by 1 each time a new record is added. To ensure that the primary key field cannot be null, we must add the NOT NULL setting to the field.

Example

 

CREATE TABLE Persons 
(
personID int NOT NULL AUTO_INCREMENT,
PRIMARY KEY(personID),
FirstName varchar(15),
LastName varchar(15),
Age int
)

 

 

Shashi Ray


     

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