Primary Key Vs Unique Key

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.

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.

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.

ALTER TABLE EMPLOYEETABLE ADD CONSTRAINT KEY_CONSTRAINT PRIMARY KEY (IDVAL)

Regards,

Venkatesan Prabu . J