Field rules govern how data can be entered into a database, and help to ensure that data entry is accurate, consistent, and complete. Using Field Rules, you describe the data that will be entered. Field rules apply to tables, not to sets.
A Field Rule is often based on the expectations and assumptions you have already made about your data. Field rules can be used to describe how entered data should be formatted, how one piece of information relates to another, and how one value can be computed from another. They can also be used to determine which values are valid. Editing Field Rules:
For example, here are some common statements that can describe the rules for a field:
The first letter in a name should be uppercase.
State abbreviations are two letters, both uppercase.
Phone numbers should include an area code, and are in the format: (617) 555-1131.
The city and state of an address can be determined from the zip code by looking them up in a zip code database.
The sale price must not be greater than the retail price.
A person's age can be computed by subtracting their birth date from the current date.