Relational database systems enforce $\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_$ rules to ensure that relationships between coupled tables remain consistent. For example, in the following database of four tables, the foreign key Supplier$\_$Number links the PART table to the SUPPLIER table. We may not add a new record to the PART table for a part with supplier number $8266$ unless there is a corresponding record in the SUPPLIER table for supplier number $8266.$ We must also delete the corresponding record in the PART table if we delete the record in the SUPPLIER table for supplier number $8266.$ In other words, we shouldn't have parts from non$-$existent suppliers!