Can we insert NULL in place of foreign key.

I have a couple of tables in my database (SQL server 2000)
1. Student -- Columns are as
"StudentID" -- INT
"StudentName" -- NVARCHAR
"DepartmentID" -- INT
2. Department -- Columns are as
"DepartmentID" -- INT
"DepartmentName" -- NVARCHAR
So in the table "Student", the field "DepartmentID" is a foreign key. Now I
tried out turning "Allow Null" on for the column "DepartmentID" in the
"Student" table. As soon as I did this, I was able to insert a row with
"DepartmentID" as <NULL>.
Now isn't this wrong? Or is it that the database overrides the checking of
the foreign key against the table "Department" when I check the "Allow Null"
option ON?
It would be really helpful if someone could elaborate on this.
Locate me in the blogosphere: http://spaces.msn.com/aayushpuriHi
Yes, referencing table may accept NULL's. Like people that don't have
children.
"Aayush Puri" <aayushpuri @. h o t m a i l . c o m> wrote in message
news:C5A2D39D-5C28-4661-B68B-B2CA6BA48CDF@.microsoft.com...
>I have a couple of tables in my database (SQL server 2000)
> 1. Student -- Columns are as
> "StudentID" -- INT
> "StudentName" -- NVARCHAR
> "DepartmentID" -- INT
> 2. Department -- Columns are as
> "DepartmentID" -- INT
> "DepartmentName" -- NVARCHAR
> So in the table "Student", the field "DepartmentID" is a foreign key. Now
> I
> tried out turning "Allow Null" on for the column "DepartmentID" in the
> "Student" table. As soon as I did this, I was able to insert a row with
> "DepartmentID" as <NULL>.
> Now isn't this wrong? Or is it that the database overrides the checking of
> the foreign key against the table "Department" when I check the "Allow
> Null"
> option ON?
> It would be really helpful if someone could elaborate on this.
> --
> Locate me in the blogosphere: http://spaces.msn.com/aayushpuri|||Aayush Puri wrote:
> I have a couple of tables in my database (SQL server 2000)
> 1. Student -- Columns are as
> "StudentID" -- INT
> "StudentName" -- NVARCHAR
> "DepartmentID" -- INT
> 2. Department -- Columns are as
> "DepartmentID" -- INT
> "DepartmentName" -- NVARCHAR
> So in the table "Student", the field "DepartmentID" is a foreign key. Now
I
> tried out turning "Allow Null" on for the column "DepartmentID" in the
> "Student" table. As soon as I did this, I was able to insert a row with
> "DepartmentID" as <NULL>.
> Now isn't this wrong? Or is it that the database overrides the checking of
> the foreign key against the table "Department" when I check the "Allow Nul
l"
> option ON?
> It would be really helpful if someone could elaborate on this.
> --
> Locate me in the blogosphere: http://spaces.msn.com/aayushpuri
SQL's foreign key constraints work like check constraints.
Specifically, values are permitted as long as the constraint isn't
violated. A null comparison returns an UNKNOWN result rather than a
FALSE one and UNKNOWN isn't regarded as a violation of the constraint.
So the behaviour you mentioned is "right" according to the SQL
standard. Whether it makes good logical sense is a different matter.
I'd suggest that you either disallow nulls or that you create some
default value in the Depatrtments table to represent the case that you
would otherwise have used a null for - a "Not Applicable" department
for example.
David Portas, SQL Server MVP
Whenever possible please post enough code to reproduce your problem.
Including CREATE TABLE and INSERT statements usually helps.
State what version of SQL Server you are using and specify the content
of any error messages.
SQL Server Books Online:
http://msdn2.microsoft.com/library/ms130214(en-US,SQL.90).aspx
--