title: database: primary key, Clustered or NonClustered
body: <p>I am creating a database in SQL server 2008,</p>
  
  <pre><code>CREATE TABLE Users
  (
      U_Id INT NOT NULL
      FirstName VARCHAR(50) NOT NULL,
      LastName VARCHAR(50) NOT NULL,
      Email VARCHAR(200)
      Password VARCHAR(50)
  )
  </code></pre>
  
  <p>I want to make U_Id the primary key.
  I would like to ask what is the difference between</p>
  
  <pre><code> CONSTRAINT pk_UserID PRIMARY KEY (U_Id)
  </code></pre>
  
  <p>this</p>
  
  <pre><code> CONSTRAINT pk_UserID PRIMARY KEY CLUSTERED (U_Id)
  </code></pre>
  
  <p>and this</p>
  
  <pre><code>CONSTRAINT pk_UserID PRIMARY KEY NONCLUSTERED (U_Id)
  </code></pre>
  
  <p>When to use each?</p>
  
  <p>I read some article but it is still unclear to me. Can someone give me a quick explanation?</p>