Defining a Database Domain

Ensure Your Data's Integrity

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A database domain, at its simplest, is the data type used by a column in a database. This data type can be a built-in type (such as an integer or a string) or a custom type that defines constraints on the data.

Data Entry and Domains

When you enter data into an online form of any kind — whether it's just your name and email, or a complete job application — a database stores your input behind the scenes.

That database evaluates your entries based on a set of criteria. For example, if you enter a zip code, the database expects to find five numbers, or for a complete U.S. zip code: five numbers followed by a hyphen, and then four numbers. If you enter your name into a zip code field, the database will likely complain.

That’s because the database is testing your entry against the domain defined for the zip code field. A domain is basically a data type that can include optional restrictions.

Understanding a Database Domain

To understand a database domain, let’s consider a few other aspects of a database:

  • A database schema defines a set of attributes, also called columns or fields.  For example, if you have a table “Employee Contact Information,” it might include attributes for FirstName, LastName, JobTitle, StreetAddress, City, State, ZipCode, PhoneNumber and Email.
  • Each attribute has a domain that defines allowable values. This could include its data type, length, values and other details.

    For example, the domain for an attribute ZipCode might specify a numeric data type, such as an integer, usually called an INT or an INTEGER, depending on the database. Or a database designer might choose to define it instead as a character, usually called a CHAR.  The attribute can be further defined to require a specific length, or whether an empty or unknown value is allowed.

    When you gather together all the elements that define a domain, you end up with a customized data type, also called a “user-defined data type” or a UDT.

    About Domain Integrity

    The allowed values of an attribute create domain integrity, which ensures that all data in a field contains valid values. 

    Domain integrity is defined by:

    • The data type, such as integer, character or decimal
    • The allowed length of the data
    • The range, defining the upper and lower boundaries
    • Any constraints, or limitations on allowable values. For example, a U.S. zip code field might enforce a complete ZIP+4 code, or a full nine-digit code.
    • The type of NULL support, or whether an attribute can have an unknown, or NULL value
    • The default value, if any
    • The date format, if applicable (for instance, dd/mm/yy or mm/dd/yyyy)

    Creating a Domain

    For databases that use SQL (Structured Query Language) or a flavor of SQL, use the CREATE DOMAIN SQL command.

    For example, the execution statement here creates a ZipCode attribute of data type CHAR with five characters. A NULL, or unknown value, is not allowed.  The range of the data must fall between “00000” and “99999.”creates a ZipCode attribute of data type CHAR with five characters. A NULL, or unknown value, is not allowed.

      The range of the data must fall between “00000” and “99999.”


    Every type of database provides a way to define a set of restrictions and rules that govern allowable data, even if it does not call it a domain. See your database’s documentation for details.

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    Chapple, Mike. "Defining a Database Domain." ThoughtCo, Aug. 14, 2017, Chapple, Mike. (2017, August 14). Defining a Database Domain. Retrieved from Chapple, Mike. "Defining a Database Domain." ThoughtCo. (accessed January 24, 2018).