What is Unique Key?

In this post, I will explain What is Unique Key?

Unique Key provides uniqueness for a column or set of columns in a table like a primary key. It is used to prevent duplicate values in a column. A database table can have more than one Unique Key and it allows at least one NULL value in a column( Nullable). Unique Key generates a Unique Non-Clustered Index

Example of Unique Key-

Create a Unique Key while creating a table
CREATE TABLE tbl_Employee
(
    EmpId INT NOT NULL,
    EmpName VARCHAR(100) NULL,
    CONSTRAINT UC_EmpId UNIQUE (EmpId)
)

Create Unique Key using alter table statement
ALTER TABLE tbl_Employee
ADD CONSTRAINT UC_EmpId UNIQUE (EmpId)

Drop Unique Key using alter table statement
ALTER TABLE tbl_Employee
DROP CONSTRAINT  UC_EmpId