title: MySQL Foreign Key Constraint - Integer Column
body: <p>I have an integer column which I would like to add a foreign key constraint to. Only problem is if that column does not have/need a value, by default MySQL puts in a '0' value. This obviously breaks the foreign key constraint as there is no record in the primary table with a PK of 0.</p>
  
  <p>How can I overcome this problem?</p>