From: "Paul DuBois" > >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? > > > > If your PRIMARY KEY would allow NULL values, it would not be possible > >> to address these rows with NULL values (*) and therefore it would not > >> be a real primary key, by definiton. It would be a unique key. > > > >That would be true for the entire key, but not for each part of the key... > > You have it backwards. Any individual column in a primary key might > contain duplicates. But the combination of values in all of the columns > must be unique. Which is the case! The same key definition would be valid as a UNIQUE key plus no combined key value is NULL. It just seems a matter of a definition in the SQL standard... Regards, Jigal. -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
