You cannot do conditional FK but it may work if column is NULL, not empty. If you want to get fancy, you have to write a trigger instead of FK.

>Hi,
>
>How common is it to change the FK constraint that would ignore empty records. Here is an example of FK:
>

>ALTER TABLE [dbo].[eq_file]  WITH CHECK ADD CONSTRAINT [FK_eq_file_site_category] FOREIGN KEY([SITE_NO],[category])
>	REFERENCES [dbo].[calpmcat] ([SITE_NO],[CATEGORY])
>	ON UPDATE CASCADE
>	ON DELETE SET DEFAULT
>

>
>The problem with the above is that table EQ_FILE could have any number of records where the field CATEGORY is empty string. Which means that the CALPMCAT has to have a record with empty CATEGORY for every SITE_NO that exists. Which does not make sense to me.
>And the only way I see to get around it is to change the above expression to ignore records with empty CATEGORY.
>My questions:
>1. How do you modify the expression to ignore empty field?
>2. Is this a good/common practice?
>
>TIA