unique key
snippet in sql

unique key

user3319

CREATE TABLE students  
(  
S_Id int NOT NULL,  
LastName varchar (255) NOT NULL,  
FirstName varchar (255),  
City varchar (255),  
UNIQUE (S_Id)  
)  

unique key

user2805

Unique Key:
Only unique value and also can contain NULL

unique key

user3603

ALTER TABLE students  
ADD UNIQUE (S_Id)