What is a unique key code example

Example 1: unique key

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

Example 2: unique key

Unique Key:
Only unique value and also can contain NULL

Example 3: unique key

ALTER TABLE students  
ADD UNIQUE (S_Id)