Remove existing SKU # from database

I deleted a product from the database because of errors within WordPress itself. When I started adding the products back into the system it would say that I needed a unique SKU #.
How do I find the SKU #s for the products inside WordPress that were deleted from my store so I can delete those SKU#s.
I found a way around by adding a .1 at the end of my SKU# but that is causing problems when I import the order into my inventory software.