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Differences Between Primary Key and Unique Key


Both Primary Key and Unique Key enforces the Uniqueness of columns in table.

Primary Key
Unique Key
Primary key does not allow NULL values.
Because of Primary Key=Unique Key+ NOT NULL		 Unique key allows NULL Values.
But it allows only single NULL value
When we creating the primary key on a table automatically cluster index is created on table. When we the Unique key on a table automatically Unique non-clustered index created on table.
A table has a only one primary key.
But Primary key can created the multiple columns this is known as composite primary key.		 A table has more than one unique key.
Syntax for Primary key On single Column.


CREATE TABLE dbo.Student
(
Id INT NOT NULL PRIMARY KEY,
FirstName VARCHAR(100),
LastName VARCHAR(100),
City VARCHAR(50)
)



 Syntax for Unique key On single Column.


CREATE TABLE dbo.Student
(
Id INT NOT NULL UNIQUE ,
FirstName VARCHAR(100),
LastName VARCHAR(100),
City VARCHAR(50)
)
Syntax for Primary key On Multiple Column.


CREATE TABLE dbo.Student
(
Id INT NOT NULL ,
FirstName VARCHAR(100),
LastName VARCHAR(100),
City VARCHAR(50),
Constraint Pk_Student primary key(Id,FirstName)
)		 Syntax for Primary key On Multiple Column.

CREATE TABLE dbo.Student
(
Id INT NOT NULL ,
FirstName VARCHAR(100),
LastName VARCHAR(100),
City VARCHAR(50),
Constraint Uk_Student Unique(Id,FirstName)
)
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