what is a unique key in sql 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: what is unique key in sql

Unique Key:
Only unique value and also can contain NULL

Example 3: unique key

ALTER TABLE students  
ADD UNIQUE (S_Id)

Example 4: primary key and unique key in sql

What is the difference between UNIQUE and PRIMARY KEY constraints? 
There should be only one PRIMARY KEY 
in a table whereas there can be
any number of UNIQUE Keys.
PRIMARY KEY doesn’t allow NULL
values whereas Unique key allows NULL values.