sql server programming: Foreign Key OR 0 -- sql server programming

You're in the

sql server programming

group:

Foreign Key OR 0

Michael Carr
10/29/2003 11:34:10 PM

sql server programming: I'd like to set up a constraint on a field such that it must be either zero
or equal to a foreign key. Is there a way to do this in SQL Server?

Thanks!
Michael Carr

Re: Foreign Key OR 0

Louis Davidson
10/30/2003 10:35:18 AM

Why? You could use null for this, but null really signifies that you will
possibly maybe have a value, just it isn't set yet.

Instead of having a zero pointing at nothing, create a foreign key value
that says 'None' with a value of 0. Now you are telling users of this data
that no way do you expect to have a different value of this key.

--
----------------------------------------------------------------------------
-----------
Louis Davidson (drsql@hotmail.com)
Compass Technology Management

Pro SQL Server 2000 Database Design
http://www.apress.com/book/bookDisplay.html?bID=266

Note: Please reply to the newsgroups only unless you are
interested in consulting services. All other replies will be ignored :)

[quoted text, click to view]

Re: Foreign Key OR 0

Andrew Pozolotin
10/30/2003 11:10:30 AM

Michael,
you can use NULL or PRIMARY KEY value

Example -

create table test (
a int primary key,
b varchar(10)
)
go

create table test2(
c int references test(a),
d varchar(10)
)
go

insert into test values (1, 'a')
insert into test values (2, 'b')
insert into test values (3, 'c')
insert into test values (4, 'd')

insert into test2 values (1, 'd')
insert into test2 values (null, 'r')
insert into test2 values (10, 'r') -- ERROR

drop table test
drop table test2

Andrew

[quoted text, click to view]

Re: Foreign Key OR 0

Michael Carr
11/5/2003 11:37:57 PM

One drawback with your solution is that you can't simultaneously enforce
constraint "0 or TableA" on Table B and constraint "TableA" on Table C. With
the NULL solution, this becomes possible because you can simply allow or
disallow NULL values on the child tables.

Michael Carr

[quoted text, click to view]

Re: Foreign Key OR 0

Louis Davidson
11/6/2003 12:00:48 AM

True, but it would be just as easy to enforce this in a check constraint
that says value <> 0.

--
----------------------------------------------------------------------------
-----------
Louis Davidson (drsql@hotmail.com)
Compass Technology Management

Pro SQL Server 2000 Database Design
http://www.apress.com/book/bookDisplay.html?bID=266

Note: Please reply to the newsgroups only unless you are
interested in consulting services. All other replies will be ignored :)

[quoted text, click to view]

Got something to say? Click here to post a reply to this thread.

Don't see what you're looking for? Try a search.

View other messages in the sql server programming group.

View Other Months
January 2003
March 2003
April 2003
May 2003
June 2003
July 2003
August 2003
September 2003
October 2003
November 2003
December 2003
January 2004
February 2004
March 2004
April 2004
May 2004
June 2004
July 2004
August 2004
September 2004
October 2004
November 2004
December 2004
January 2005
February 2005
March 2005
April 2005
May 2005
June 2005
July 2005
August 2005
September 2005
October 2005
November 2005
December 2005
January 2006
February 2006
March 2006
April 2006
May 2006
June 2006
July 2006
August 2006
September 2006
October 2006
November 2006
December 2006
January 2007
February 2007
March 2007
April 2007
May 2007
June 2007
July 2007
August 2007
September 2007
October 2007
November 2007
April 2008
August 2008

home · blog · groups

Questions? Comments? Contact the dn