Friday, August 30, 2013

SQL SERVER PRIMARY KEY Constraint

1

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

2

Primary keys must contain unique values.

3

A primary key column cannot contain NULL values.

4

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

 

 

SQL PRIMARY KEY Constraint on CREATE TABLE

The following SQL creates a PRIMARY KEY on the "P_Id" column when the "Persons" table is created:

 

CREATE TABLE Persons

(

P_Id int NOT NULL PRIMARY KEY,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255)

)

 

To allow naming of a PRIMARY KEY constraint, and for defining a PRIMARY KEY constraint on multiple columns, use the following SQL syntax:

 

CREATE TABLE Persons

(

P_Id int NOT NULL,

LastName varchar(255) NOT NULL,

FirstName varchar(255),

Address varchar(255),

City varchar(255),

CONSTRAINT pk_PersonID PRIMARY KEY (P_Id,LastName)

)

 

Note: In the example above there is only ONE PRIMARY KEY (pk_PersonID). However, the value of the pk_PersonID is made up of two columns (P_Id and LastName).


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