SQL Constraints - Unique Key Constraint

The Unique key constraint is to maintain a table column with unique values. Unlike Primary Key, we can add the unique key to more than one column as well unique key allows one null value to be entered in the column.

CREATE TABLE Employees(

    ID int NOT NULL Primary Key,

    LastName varchar(255) NOT NULL UNIQUE,

    FirstName varchar(255),

    Age int

);

Add Unique Key to an existing table  using alter command

ALTER TABLE Employees
ADD UNIQUE (ID);

Add the unique key to more than one column 

ALTER TABLE Employees
ADD CONSTRAINT UC_ Employees UNIQUE (ID,LastName);

Drop Unique Key 

ALTER TABLE Employees
DROP UNIQUE UC_ Employees;

Difference between Unique Key and Primary Key

Primary Key	Unique Key
Primary Key does not allow null values to be inserted	Unique Key allows one null value to be entered into one column
We can't add Primary Key to, more than one column in one table. ()	We can add Unique Key to more than one column under a table.