title: Select non existing Numbers from Table each ID
body: <p>I‘m new in learning TSQL and I‘m struggling getting the numbers that doesn‘t exist in my table each ID.</p>
  
  <p>Example:</p>
  
  <pre><code>CustomerID Group
  1          1
  3          1
  6          1       
  4          2
  7          2
  </code></pre>
  
  <p>I wanna get the ID which does not exist and select them like this</p>
  
  <pre><code>CustomerID Group
  2          1       
  4          1
  5          1
  5          2
  6          2
  ....
  </code></pre>
  
  <p>..</p>
  
  <p>The solution by usin a cte doesn‘t work well or inserting first the data and do a not exist where clause.</p>
  
  <p>Any Ideas?</p>