Wrox Programmer Forums
| Search | Today's Posts | Mark Forums Read
SQL Server 2000 General discussion of Microsoft SQL Server -- for topics that don't fit in one of the more specific SQL Server forums. version 2000 only. There's a new forum for SQL Server 2005.
Welcome to the p2p.wrox.com Forums.

You are currently viewing the SQL Server 2000 section of the Wrox Programmer to Programmer discussions. This is a community of software programmers and website developers including Wrox book authors and readers. New member registration was closed in 2019. New posts were shut off and the site was archived into this static format as of October 1, 2020. If you require technical support for a Wrox book please contact http://hub.wiley.com
  #1 (permalink)  
Old September 26th, 2003, 10:13 AM
Registered User
 
Join Date: Sep 2003
Location: , , .
Posts: 8
Thanks: 0
Thanked 0 Times in 0 Posts
Default Foreign Key Problem

I have a general question. Can a Foreign key field take null values? I read at a couple of places that it can , but somme people say that it can't since a FK should be a values referring some PK(makes sense). I am confused. I am designing a database and I need to know soon. Thanks for your help in advance.............

  #2 (permalink)  
Old September 26th, 2003, 10:58 AM
Friend of Wrox
 
Join Date: Jun 2003
Location: Hudson, MA, USA.
Posts: 839
Thanks: 0
Thanked 1 Time in 1 Post
Default

A column in a base table which is a foreign key in another table can indeed be null. If there is a referential constraint on the column, then values in that column must be found in the referenced table. But if there is no value in the base column (i.e. NULL), then that is perfectly OK - it doesn't violate the constraint, because NULL means there is no value, so there is nothing to check.

BTW, it is not requirement that the foreign key constraint refer to the primary key of the foreign table, but it usually does. As long as the constraint refers to a column (or columns) which has a UNIQUE constraint defined, the reference will be allowed.

Jeff Mason
Custom Apps, Inc.
www.custom-apps.com


Similar Threads
Thread Thread Starter Forum Replies Last Post
Foreign key not updating with Primary key xavier1945 BOOK: Access 2003 VBA Programmer's Reference 2 July 4th, 2007 09:48 PM
Composite Foreign Key bijgupt SQL Server 2005 6 May 4th, 2007 08:46 AM
foreign key constraints - referential integrity DTSDeveloper SQL Server DTS 0 September 6th, 2005 01:10 PM
FOREIGN KEY and PRIMARY KEY Constraints junemo Oracle 10 June 15th, 2004 01:00 AM





Powered by vBulletin®
Copyright ©2000 - 2020, Jelsoft Enterprises Ltd.
Copyright (c) 2020 John Wiley & Sons, Inc.