A domain is a user-defined data type that, together with other attributes, can restrict the range of acceptable values or provide defaults. A domain extends one of the built-in data types. Normally, the range of permissible values is restricted by a check constraint. In addition, a domain can specify a default value and may or may not allow NULLs.
Defining your own domains provides many benefits including:
Preventing common errors if inappropriate values are entered. A constraint placed on a domain ensures that all columns and variables intended to hold values in a range or format can hold only the intended values. For example, a data type can ensure that all credit card numbers typed into the database contain the correct number of digits.
Making the applications and the structure of a database easier to understand.
Convenience. For example, you may intend that all table identifiers are positive integers that, by default, auto-increment. You could enforce this restriction by entering the appropriate constraints and defaults each time you define a new table, but it is less work to define a new domain, then simply state that the identifier can take only values from the specified domain.
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