After setting a validation rule, what must you do to apply it to the database?

To apply a validation rule to the database, it is essential to save the table after making the changes. This action ensures that all modifications, including the new validation rule, are stored in the database. Simply confirming the changes by clicking 'OK' does not commit the changes to the database; it might just close the properties window.

Saving the table captures the complete set of alterations, ensuring that the validation rule operates as intended in future data entries. This process also allows the rule to be enforced across all relevant operations within the table or form where it is applicable. Thus, the key step is saving the table to properly apply and enforce the validation rule throughout the database.