A unique constraint will only allow one NULL value. If you must enable support for multiple nulls, you'll have to enforce the uniqueness using triggers.

>> It is possible to write a line in the header table without adding lines in the detail.

Yes. The FK constraint should be created on the Detail table.

-Mike

>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