> > A primary key absolutely forbids duplicate values. > > > > Indexes created with the UNIQUE keyword do not allow duplicates, except > > for the special case that multiple NULL values are allowed. > > I realise that it may (and is) defined in such a way, but it still does not > explain *why* part of a PRIMARY key might not be NULL.
Because a value in a PK should uniquely identify a row. Using one or more NULLs negates this... The best systems don't use NULLs at all. NULL is an abomination. You should only store "values" that are "true". Eg: employee with number 1 has a name of "Martijn". If "salary" is optional, do not make it nullable, but create a separate table "employee_salaries". This way, there's never NULL confusion either. With regards, Martijn Tonies Database Workbench - developer tool for InterBase, Firebird, MySQL & MS SQL Server Upscene Productions http://www.upscene.com -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
