title: selecting random rows with normal distribution based on a column in SQL Server 2012
body: <p><strong>FULL DETAILS:</strong>
  let me explain more clear. this is a table including about 100 question. every question has a BooKRange property that shows from which part of the book, this question hast fetched with values 1,2,3,4. and there is another property called Level that shows level of the difficulty of the question with values 1,2,3,4,5. now i need to randomly select 20 question that have to include all four Book Ranges and all five levels with a normal distribution.</p>
  
  <p><strong>please consider that i need to select distinct rows.</strong></p>
  
  <p>thank you very much.</p>
  
  <p>edit: added the table</p>
  
  <pre><code>CREATE TABLE [dbo].[Question] (
      [QuesID]    INT            IDENTITY (1, 1) NOT NULL,
      [BookRange] NVARCHAR (50)  NULL,
      [Level]     NVARCHAR (50)  NULL,
      PRIMARY KEY CLUSTERED ([QuesID] ASC)
  );
  </code></pre>