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Defines a new domain.


CREATE DOMAIN name [AS] data_type [DEFAULT expression] 
       [CONSTRAINT constraint_name
       | NOT NULL | NULL 
       | CHECK (expression) [...]]


CREATE DOMAIN creates a new domain. A domain is essentially a data type with optional constraints (restrictions on the allowed set of values). The user who defines a domain becomes its owner. The domain name must be unique among the data types and domains existing in its schema.

Domains are useful for abstracting common constraints on fields into a single location for maintenance. For example, several tables might contain email address columns, all requiring the same CHECK constraint to verify the address syntax. It is easier to define a domain rather than setting up a column constraint for each table that has an email column.


The name (optionally schema-qualified) of a domain to be created.
The underlying data type of the domain. This may include array specifiers.
DEFAULT expression
Specifies a default value for columns of the domain data type. The value is any variable-free expression (but subqueries are not allowed). The data type of the default expression must match the data type of the domain. If no default value is specified, then the default value is the null value. The default expression will be used in any insert operation that does not specify a value for the column. If a default value is defined for a particular column, it overrides any default associated with the domain. In turn, the domain default overrides any default value associated with the underlying data type.
CONSTRAINT constraint_name
An optional name for a constraint. If not specified, the system generates a name.
Values of this domain are not allowed to be null.
Values of this domain are allowed to be null. This is the default. This clause is only intended for compatibility with nonstandard SQL databases. Its use is discouraged in new applications.
CHECK (expression)
CHECK clauses specify integrity constraints or tests which values of the domain must satisfy. Each constraint must be an expression producing a Boolean result. It should use the key word VALUE to refer to the value being tested. Currently, CHECK expressions cannot contain subqueries nor refer to variables other than VALUE.


Create the us_zip_code data type. A regular expression test is used to verify that the value looks like a valid US zip code.

       ( VALUE ~ '^\\d{5}$' OR VALUE ~ '^\\d{5}-\\d{4}$' );


CREATE DOMAIN conforms to the SQL standard.