That's one of the kickers. The 7% of the time the column is populated is determined by business logic. And when the business logic says it's needed, at application run time if certain conditions were met, the column takes on the characteristic NOT NULL attribute.
-----Original Message----- From: Dan Shirah [mailto:[EMAIL PROTECTED] Sent: Wednesday, May 02, 2007 3:50 PM To: Max Thayer Cc: email@example.com Subject: Re: [PHP-DB] DB Design Concepts Max, I am assuming that since column b will only be populated 7% of the time that it is not a value specific column (does not matter if it has a value or not) Therefore I would suggest leaving the NULL's in there as it will not (at least should not) affect any system performance. On 5/2/07, Max Thayer <[EMAIL PROTECTED]> wrote: 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> <http://www.hwi.buffalo.edu/> -- PHP Database Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php