Appropriate values for attributes
When you design a database, you need to decide what values are acceptable for the various attributes of an entity.
For example, you would not want to allow numeric data in an attribute for a person's name. The data types that you choose limit the values that apply to a given attribute, but you can also use other mechanisms. These other mechanisms are domains, null values, and default values.
The following are the appropriate values for attributes of an entity:
Domain
A domain describes the conditions that an attribute value must meet to be a valid value. Sometimes the domain identifies a range of valid values. By defining the domain for a particular attribute, you apply business rules to ensure that the data will make sense.
Example 1: A domain might state that a phone number attribute must be a 10-digit value that contains only numbers. You would not want the phone number to be incomplete, nor would you want it to contain alphabetic or special characters and thereby be invalid. You could choose to use either a numeric data type or a character data type. However, the domain states the business rule that the value must be a 10-digit value that consists of numbers.
Example 2: A domain might state that a month attribute must be a 2-digit value in the range 01–12. Again, you could choose to use datetime, character, or numeric data types for this value, but the domain demands that the value must be in the range of 01 through 12. In this case, incorporating the month into a datetime data type is probably the best choice. This decision should be reviewed again during physical database design.
Null values
When you are designing attributes for your entities, you will sometimes find that an attribute does not have a value for every instance of the entity. For example, you might want an attribute for a person's middle name, but you can't require a value because some people have no middle name. For these occasions, you can define the attribute so that it can contain null values.
A null value is a special indicator that represents the absence of a value. The value can be absent because it is unknown, not yet supplied, or nonexistent. The DBMS treats the null value as an actual value, not as a zero value, a blank, or an empty string.
Just as some attributes should be allowed to contain null values, other attributes should not contain null values.
Example: For the EMPLOYEE entity, you might not want to allow the attribute EMPLOYEE_LAST_NAME to contain a null value.
Default values
In some cases, you may not want a given attribute to contain a null value, but you don't want to require that the user or program always provide a value. In this case, a default value might be appropriate.
A default value is a value that applies to an attribute if no other valid value is available.
Example: Assume that you don't want the EMPLOYEE_HIRE_DATE attribute to contain null values and that you don't want to require users to provide this data. If data about new employees is generally added to the database on the employee's first day of employment, you could define a default value of the current date.