Jigal, create table YourTable ( id INT(11), name VARCHAR(32), value INT(11), PRIMARY KEY(id,name,value) )
let's assume that PRIMARY KEY works like you want (accept NULLs) and we have a row in your table: (id,name,value) = (1,NULL,12) Then you insert a new row: insert into YourTable (id,name,value) values (1,NULL,12). Before inserting MySQL will try to find a record with the same values. But since comparison with NULL value returns always FALSE MySQL will think that there is no such record in the table. After this point you will get 2 identical records in the table. Mikhail. ----- Original Message ----- From: "Jigal van Hemert" <[EMAIL PROTECTED]> To: "Dawid Kuroczko" <[EMAIL PROTECTED]> Cc: <mysql@lists.mysql.com> Sent: Tuesday, April 26, 2005 4:47 PM Subject: Re: why NOT NULL in PRIMARY key?? > From: "Dawid Kuroczko" > > > > It can't have anything to do with the 'uniqueness' of the data, since I > can > > > have a lot of 'zero'-values in the column, as long as the combination of > > > columns in the PRIMARY key results in unique values. > > > > Because it is a PRIMARY KEY. I mean phrase 'PRIMARY KEY' means "a key > > with which each row can be explicitly addressed". So if you have 2000 > > rows in a table, you can write 2000 SELECT statemens which will use > > columns in primary key and each of these SELECT statements will > > return exactly one (different) row. > With the NULL values included it will still uniquely identify each row... > > 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. > > > 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... > > Regards, Jigal. > > > -- > MySQL General Mailing List > For list archives: http://lists.mysql.com/mysql > To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED] > -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
