Cindy tried to delete a customer but the customer still had open invoices so the system would not allow her to delete the customer. More than likely was enforced.

The answer is "Referential integrity".

Explanation:

This relates to one aspect of data integrity. Data across multiple tables are linked via relationships. In view of the distinct customer & invoice tables, each invoice table should have the primary key specified throughout the Customer table, for instance. Whenever the invoice table contains a customer ID reference. This ID including its customer will be only a table of the customer.

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