> > >I would understand it if it would mean that the key as a whole could not > be > > >NULL, but the restriction that each column that is part of a PRIMARY KEY > > >must have the NOT NULL constraint is not logical. > > > > Sure it is. If any part could be NULL, then it could contain duplicate > > NULL values, thus compromising uniqueness of that part, and of the entire > > key as well. > > > > Suppose you have a two part key on columns a and b, with b allowed to > > be NULL. Then you could have these values: > > > > x y > > x NULL > > x z > > x NULL > > > > Now, how do you uniquely identify the 2nd and 4th rows? > > > The same is true for any other value... Now that the columns have a NOT NULL > constraint the records that previously contained NULL now hold '0'. > > x y > x 0 > x z > x 0 > > Now, how do you uniquely identify the 2nd and 4th rows?
Yes well, exactly the point: this is not possible in a primary key :-) 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]
