Sometimes the data is not what you want it to be. Or rather the quality is not what you want it to be. What I am referring to in this case is when you have a primary key and you want to reference that key from another table by a foreign key constraint but for some reason the foreign key is NULL and you don’t want NULL as a primary key in that table. Sure you can add a dummy row manually to your table with a value of '-1' or whatever you like. But what if you TRUNCATE your table before each load? Should you add the row manually every time? I’m sure that there’s hundreds of solutions  to this problem but below is how I choose to solve my particular problem.