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
 

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Difference between Primary Key and Unique Key?

In this post, I will explain the difference between Primary Key and Unique Key.

Primary Key and Unique Key provide uniqueness for a column or set of columns in a table. But there are different characteristics or behaviour of them, which makes Primary Key and Unique Key different. Please find below the difference between Primary Key and Unique Key :

Sr No	Primary Key	Unique Key
1	A database table can have only one Primary Key.	A database table can have more than one Unique Key.
2	Primary Key is not nullable (Doesn't allow NULL value).	Unique Key is nullable (Allows at least one NULL value).
3	Primary Key generates a Unique Clustered Index.	Unique Key generates a Unique Non-Clustered Index.
4	Primary Key is used to identify a row (a record) in a table.	Unique Key is used to prevent duplicate values in a column.

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