Some data formats are just hopeless to work with. The last few weeks, I've had the "pleasure" of working with an Axapta database. (Microsoft Dynamics Axapta). More spesifically, working on accounts in Axapta. For some reason, this field looks like it's numeric, but no. It's alphanumeric, which means sorting and functions like "between" has to be used very carefully. And just to be a bit more confusing, the field is what the Axapta-people calls "right aligned". This means that the the column is a 10 character varchar, and the first x spaces are blank, with the actual account number value in the back. So a field that looks like this
3101,
it's probably more like this
' 3101'.
And '3101' is not the same as ' 3101', if you would want to compare these two. And a full 10 blanks is not the same as NULL either, if you would want to find all the empty ones.
So, if you want to have a remote chance of ever using your data for something usefull, like filtering, make sure all columns have a decent dateformat, and make sure that there's something that can be used for filtering in each column... NULL is definately preferable to "one or more blank spaces and maybe some characters if you're lucky".