From: Rik Wasmus
Date: June 7 2011 3:02pm
Subject: Re: Data missing after field optimization
List-Archive: http://lists.mysql.com/mysql/225200
Message-Id: <201106071702.28045.rik@grib.nl>
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> On Jun 6, 2011, at 10:06 PM, Johan De Meersman wrote:
> > What exactly do you mean by "removing the NULL default"?  Did you set
> > your colums NOT NULL?
> 
> 	Yes.  That's all I did.

In stead of getting info drop-by-drop, you might want to share the output of 
SHOW CREATE TABLE...,, but my guess is you had a UNIQUE key somewhere that got 
violated when converting to NULL's to empty strings. If would require an ALTER 
IGNORE TABLE... instead of a normal ALTER TABLE though.

That, or an outside source (code we cannot see querying for NOT NULLs etc.)
-- 
Rik Wasmus