title: LEFT JOIN in MySQL using multiple ON conditions
body: <p>I know how to do a LEFT JOIN in MySQL with a single conditional or with using WHERE key IS NULL, but how do you do it when you have multiple conditions in your ON statement?  I have an invoice table that contains an Order Number and Invoice Date.  I also have a ship table that contains an Order Number and Ship Date.  I want to return any items from the invoice table where there is NOT a corresponding record in the ship table.  For example...</p>
  
  <pre><code>invoice table:
  
  InvoiceNum  OrderNum  InvoiceDate
  106433        136365      2011-10-03
  111888        136365      2012-06-19
  
  ship table:
  
  OrderNum   ShipDate
  136365       2012-06-19
  </code></pre>
  
  <p>I want to write a query that will return just invoice number 106433 from the invoice table.  Does someone know how best to do that.  I am joining other tables into the query, but this is the one that I'm having trouble figuring out.  Thanks for any help anyone can give!</p>