Answers for "unique composite key"

unique key

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

unique key

ALTER TABLE students  
ADD UNIQUE (S_Id)

unique key

Unique Key:
Only unique value and also can contain NULL