"Data domain" is a term used in database fields and data management. It refers to the range of acceptable values which a particular row or field can contain. Enforcing data domains keeps the information in a database consistent and within acceptable parameters, avoiding situations in which databases either produce nonsensical results or are unable to answer specific queries for lack of sufficient information. Defining a data domain is done by a database's programmer, who often stipulates a list of proper values for an entry field in the program.
Perhaps the easiest way to understand the concept of a data domain is by examining an example. Consider a field in an employee database into which an end user is supposed to record the employment status of individual employees. Each employee in the organization will be either a current or past employee; therefore, those are the only acceptable entries in the data domain for that field. When creating the database, the programmer would typically indicate that these are the only acceptable responses for someone entering data into the "Employee Status."
Another, more complex, example of a data domain would be a field for an employee's phone number. In the US, the data domain in this field would have to be limited to numbers only, along with a maximum length of 10 characters; in other words, an area code plus a seven digit phone number. A programmer who wanted to enforce an even tighter level of control over the data domain could program it so that only actual area codes and prefixes could be used, preventing individuals from attempting to claim (999) 999-9999 as their phone number. In either example, the limits placed on the effective data domains is accomplished by using a tool known as a reference table.
Acceptable data values are typically stored in what is known as a reference table. A reference table is a table linked to the database which contains the acceptable values or ranges of values for data for specific tables in the database. Information being entered into the database is compared against the reference table to determine whether the user has accidentally entered an invalid entry. Invalid entries are normally flagged with an error message and prompt a second opportunity to enter a correct value into the database.