DB Design Concepts

I'm using MySQL 5.x InnoDB engine, transactional tables. I have a conceptual design question. If I have a two columns 'a' and 'b', a is the primary key, and b is a type double, in table 1 (T1) for which column b will have many NULL values, do I leave it with an allow null constraint on the column or pull the column and place it into table 2 (T2) with a foreign key, making a simple optional one-to-one relationship. Over the course of time, as the table fills with records, will a column w/ many NULL values have a detrimental effect on performance or maintenance with regards to the DB? Am I missing something here in DB design 101, by leaving the column in the T1 and knowing it will only be populated 7% of the time; what are the major implications based on the RDBMS and engine I'm using? Do I go to 2nd NF simply because a column is not going to be populated as often? Max H. Thayer Lead Software Developer Center for High-Throughput Structural Biology Hauptman-Woodward Medical Research Inst. 700 Ellicott St. Buffalo, NY 14203 Phone: 716-898-8637 Fax: 716-898-8660 http://www.chtsb.org <http://www.chtsb.org/> http://www.hwi.buffalo.edu <http://www.hwi.buffalo.edu/>