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The Constraints tab of the Column property sheet has the following components:
Values can be null Select this option if values in the column can be NULL. If the column is based on a domain, you can retain the domain's nullability or override it for the column.
Values cannot be null Select this option to allow duplicate values, but not allow NULL values in this column.
Values cannot be null and must be unique Select this option if values in the column cannot be NULL and must be unique.