From: Martijn Tonies
Date: April 26 2005 2:01pm
Subject: Re: why NOT NULL in PRIMARY key??
List-Archive: http://lists.mysql.com/mysql/183154
Message-Id: <052601c54a68$6bb363e0$3802a8c0@martijnws>
MIME-Version: 1.0
Content-Type: text/plain;
	charset="iso-8859-1"
Content-Transfer-Encoding: 7bit



> Not every DBMS...
>
> MSSQL:
> Create Unique Index
> Microsoft(r) SQL Server(tm) checks for duplicate values when the index
> is created (if data already exists) and checks each time data is added
> with an INSERT or UPDATE statement. If duplicate key values exist, the
> CREATE INDEX statement is canceled and an error message giving the first
> duplicate is returned. Multiple NULL values are considered duplicates
> when UNIQUE index is created.
>
>
> SYBASE:
> Create Unique Index
> Prohibits duplicate index (also called key) values. The system checks
> for duplicate key values when the index is created (if data already
> exists), and checks each time data is added with an insert or update. If
> there is a duplicate key value or if more than one row contains a null
> value, the command is aborted and an error message giving the duplicate
> is printed.

An unique index is not a primary key constraint.

With regards,

Martijn Tonies
Database Workbench - developer tool for InterBase, Firebird, MySQL & MS SQL
Server
Upscene Productions
http://www.upscene.com