Difference Between Unique And Composite Primary key In sql server
<p>I want to know what is the difference between unique key and composite primary key in SQL Server.</p>

<p>According to w3c school:</p>

<blockquote>
  <p>The UNIQUE constraint uniquely identifies each record in a database table.</p>
  
  <p>The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns.</p>
  
  <p>A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.</p>
  
  <p>Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.</p>
</blockquote>

<p>We can create composite primary key by using this:</p>

<pre><code>CREATE TABLE Persons
(
P_Id int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
UNIQUE (P_Id)
)
</code></pre>

<p>For composite primary key syntax:</p>

<pre><code>CREATE TABLE Persons
(
P_Id int,
C_Id int,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Address varchar(255),
City varchar(255),
Primary Key (P_Id,C_Id)
);
</code></pre>