I once presided over a DB2 to Oracle migration for reasons that had nothing to do with technological considerations and everything to do with politics. The hardest thing to get right was the different interpretations of what was 'null' in a given column. It was a never ending stream of in DB2 it's null but in oracle it's not. Or vice versa. You wouldn't think you'd have so much of that to sort though but I would bet at least 40-50% of the issues we ran into were purely around what one database counted as null and another did not. The empty string in one database would null in another as an example.