A different kind of value constraint can be enforced by restricting the type of data allowed in a field. If we do not have such a constraint, we can put any type of data in the field. For example, you can have -99 as the value of the last name of a person—of course, that would be wrong. To ensure this does not happen, we can enforce the type of the last name to be a non-numeric alphabetic string. The preceding example shows a logical expression for this constraint.

A type constraint is a special kind of domain constraint. The domain of a data property or attribute is the possible set of values that are allowed for that attribute. For example, the possible values for the day part of the date field can be between 1 and 31, while a month may have the value between 1 and 12, or a value from the set January, February till December.