Constraints
A constraint is a condition placed on the fields in a database table. For example, you might require that the value in a certain numeric field be between 1 and 100.
A referential constraint is one where a field is related to another field. For example, you might require that the CustomerId field in a new Orders record have a value that matches the CustomerId field in some record in the Customers table. In other words, the database cannot use a CustomerId in the Orders table unless that CustomerId has already been defined in the Customers table. That is sensible because it would be illogical to allow an order without a customer to place the order.
Referential constraints protect the database from garbage data. The user interface ...
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