Hello

I've got the following problem:

a header and detail table, the join between them is a field called cce_no. This field is not the primary key of the header table. It is possible to write a line in the header table without adding lines in the detail. At that moment cce_no contains the null value.
Now I want to enforce data integrity between the two tables. To do so, I need to create an unique index on cce_no in the header table to enforce the constraint. When I try to do this, I get a 'duplicate key' error because of the null values in cce_no.
How can I solve this problem ??

Greetings,
Hans