A problem that can arise in a poorly designed relational database when attributes that are not characteristics of the primary key of a relation are stored in that table. The problem arises because new information about those attributes cannot be entered in the database without violating integrity rules. For example, assume that information about vendors is only stored as part of the purchases table. Data about potential new vendors, or about alternate supliers, could not be added until a purchase from them was made. Otherwise, the purchase order number column, the primary key of the purchases table, would have a null value, violating the entity integrity rule.