Unique Key constraints:

Unique key constraint will provide you a constraint like the column values should retain uniqueness.
It will allow null value in the column.
It will create non-clustered index by default
Any number of unique constraints can be added to a table.


Code:
CREATE TABLE EMPLOYEETABLE
	(EMPID INT ,
         IDVAL INT NOT NULL,  
	 FIRSTNAME VARCHAR(30) ,
	 LASTNAME VARCHAR(30) ,
	 CITY VARCHAR(30),
	 JOININGDATE DATETIME)
I have created a table EmployeeTable and i am trying add a unique constriant on the column IDVAL.

Code:
 ALTER TABLE EMPLOYEETABLE
 ADD CONSTRAINT UNIQUE_CONSTRAINT 
 UNIQUE (IDVAL)

Primary Key:
Primary key will create column data uniqueness in the table.
Primary key will create clustered index by default
Only one Primay key can be created for a table
Multiple columns can be consolidated to form a single primary key
It wont allow null values.


Code:
 ALTER TABLE EMPLOYEETABLE
 ADD CONSTRAINT KEY_CONSTRAINT 
 PRIMARY KEY (IDVAL)
Please provide me your valuable feedback regarding this article.

Regards,
Venkatesan Prabu . J

Last edited by venkatesanj@hcl.in; 12Mar2008 at 08:46.. Reason: Small changes