The other responses should get you started if this is something you
really want to do. However, I'll play devil's advocate here and just
raise the question why you would want to make this change in the first
place. I'm not quite as anti-NULL as a lot of arguments I've read
against them, but I tend to agree that the number of columns that
accept NULL values should be kept as small as possible. Even if you
decide that you need to allow NULL values in some cases, IMHO I
wouldn't write a script that ran through my entire database and opened
every column in every table to accept.
I just thought I'd throw this out there...
A lot of people who post questions on this list are programming their algorithms and structuring their applications in a certain way because that's what the client wants, or that's what their boss told them to do. Yes, accepting NULL values in a database is frowned upon (unless the table is a transaction table)... but I doubt his boss or his client cares in the least.
I don't understand what you mean about a "transaction table" - you should only use nulls if you understand what they do and why you'd need them in that particular case. I'd ask why and find specifically what they want/why they suggested it and make sure they understand the repercussions.
--
Postgresql & php tutorials
http://www.designmagick.com/