title: Insert rows if they don't exist into a table from another table
body: <p>I know how to make an insert of a row of values into a table using insert into. 
  However, I cannot find an appropriate idiom for accomplishing the task that I have. </p>
  
  <p>I have two tables. One table is the primary table and the other is an additional table. I need to insert rows that do not exist in the primary from additional table ignoring any duplicates. </p>
  
  <p>I am trying to use some variations of:</p>
  
  <pre><code>replace into primary 
  select * from additional;
  </code></pre>
  
  <p>But this obviously replaces the rows which is not what I want. Should I use <code>on duplicate</code> somehow or am I in a completely wrong direction? </p>
  
  <p>EDIT:</p>
  
  <p>Both of the columns have a unique surrogate key column which is just an integer value. </p>