Data validation makes Access test data to make sure that it conforms to what you want to appear in the table. Data validation works best in number, currency, and date/time fields. You can create a validation rule for text entries, but doing so can be complicated—especially if you want to test a lot of text variables.
There are actually two boxes that relate to data validation:
Validation Rule box: Used to specify the requirements for data entered in the field.
Validation Text box: Used to specify the message that will be displayed to the user when data that violates the validation rule is entered.
Now, whenever a user violates the validation rule, Access will display the validation text.
Consider the table below a “cheat sheet.” It contains samples of the most common types of validation rules. Feel free to copy, modify, or mix and match these examples to create your own validation rules.