Quick question re foreign keys.

I have a table where one column references by foreign key a column from 
another table. However, the column in this first table does not always 
contain data which results in a not-null constraint violation when I 
attempt an insert.

My question therefore is, is it possible to create a foreign key that is 
conditional, i.e. only enforce the foreign key where the value in that 
table is not null.

-- 
Paul Lambert
Database Administrator
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