title: Select rows with no match between two tables by sem
body: <p>I have two tables, Table1 and Table2. I want to select distinct rows in Table2 that are not contained in Table1. Here is an example:</p>
  
  <pre><code>  Table1        
  
     | A  |   | sem|
     ------------------
  1. | aa |   | 1 |
     ---------------
  2. | bb |   | 1 |
     ----------------
  3. | aa |   | 2 |
     -----------------
  4. | cc |   | 2 |
     ---------------
  
  Table2
  
     | B  |
     ------
  1. | aa |
     ------
  2. | aa |
     ------
  3. | bb |
     ------
  4. | cc |
     ------
  5. | cc |
     ------
  6. | ee |
     ------
  7. | ee |
     ------
  </code></pre>
  
  <p>I would want the those row who is not common  in  this two table  when sem = 1 
  like 
  output  for  sem =  1
    result</p>
  
  <h2>    | B |</h2>
  
  <ol>
  <li>| cc |</li>
  <li>| ee |</li>
  </ol>