what is unique key in sql

1This constraint ensures all values in a column are unique.
2Example 1 (MySQL): Adds a unique constraint to the id column when
3creating a new users table.
4CREATE TABLE users (
5id int NOT NULL,
6name varchar(255) NOT NULL,
7UNIQUE (id)
8);
9Example 2 (MySQL): Alters an existing column to add a UNIQUE
10constraint.
11ALTER TABLE users
12ADD UNIQUE (id);

1CREATE TABLE students  
2(  
3S_Id int NOT NULL,  
4LastName varchar (255) NOT NULL,  
5FirstName varchar (255),  
6City varchar (255),  
7UNIQUE (S_Id)  
8)  

1What is the difference between UNIQUE and PRIMARY KEY constraints? 
2There should be only one PRIMARY KEY 
3in a table whereas there can be
4any number of UNIQUE Keys.
5PRIMARY KEY doesn’t allow NULL
6values whereas Unique key allows NULL values.

1Unique Key:
2Only unique value and also can contain NULL

1ALTER TABLE students  
2ADD UNIQUE (S_Id)  

1CREATE TABLE Students ( 	 /* Create table with a single field as unique */
2    ID INT NOT NULL UNIQUE
3    Name VARCHAR(255)
4);
5
6CREATE TABLE Students ( 	 /* Create table with multiple fields as unique */
7    ID INT NOT NULL
8    LastName VARCHAR(255)
9    FirstName VARCHAR(255) NOT NULL
10    CONSTRAINT PK_Student
11    UNIQUE (ID, FirstName)
12);
13
14ALTER TABLE Students 	 /* Set a column as unique */
15ADD UNIQUE (ID);
16
17ALTER TABLE Students 	 /* Set multiple columns as unique */
18ADD CONSTRAINT PK_Student 	 /* Naming a unique constraint */
19UNIQUE (ID, FirstName);
20