>I just did a little test, after setting the FK (as per code below). Then I added a couple of records to the EQ_FILE with different value in SITE_NO and NULL in CATEGORY. And no problem; no violation of FK constraint. Does it mean the SQL Server automatically ignores NULL values when checking FK constraints?
>

SQL Server does not enforce FK when any child table column value in FK relationship is null. Which means, when CATEGORY is null, SITE_NO will not be enforced