title: MySQL insert unique from union
body: <p>I haven't worked much with databases before but I understand a bit of the basics. Right now, I've got two tables, we'll call Table 1 and Table 2. I'm trying to create a trigger so that when I insert data into Table 1, it fires the trigger to get all distinct values for col1 in Table 1, which I want to use to insert into Table 2. As more data gets added into Table 1, duplicates will no doubt occur. If a duplicate occurs, I don't want the duplicate inserted into Table 2. So, I thought that I might use a Union to pull the unique values and insert them into Table 2, but after the Union I'm not entirely sure what to do. Am I crazy for thinking this way and there's a way to do this better?</p>
  
  <pre><code>DELIMITER // 
  CREATE TRIGGER testdb 
  AFTER INSERT ON testdb.table1
  FOR EACH ROW
  BEGIN    
      SELECT col1 FROM table1
      UNION
      SELECT col1 FROM table2
      (I imagine the insert statement would go here)
  END //
  DELIMITER ;
  </code></pre>
  
  <p>In short, data goes into Table 1, trigger launches on insert to get unique values from Table 1 Col 1 and Table 2 Col 1 in the attempt to get all unique values, and then I want to insert the unique values into Table 2 Col 1, but some of those unique values may already exist.</p>