redhat man page for create_domain

CREATE 
DOMAIN(7)						   SQL Commands 						  CREATE DOMAIN(7)

NAME
       CREATE DOMAIN - define a new domain

SYNOPSIS
       CREATE DOMAIN domainname [AS] data_type
	   [ DEFAULT default_expr ]
	   [ constraint [, ... ] ]

       where constraint is:

       [ CONSTRAINT constraint_name ]
       { NOT NULL | NULL }

   PARAMETERS
       domainname
	      The name (optionally schema-qualified) of a domain to be created.

       data_type
	      The  underlying data type of the domain. This may include array specifiers.  Refer to the User's Guide for further information about
	      data types and arrays.

       DEFAULT
	      The DEFAULT clause specifies a default value for columns of the domain data type. The value is  any  variable-free  expression  (but
	      subselects are not allowed).  The data type of the default expression must match the data type of the domain.

	      The default expression will be used in any insert operation that does not specify a value for the column. If there is no default for
	      a domain, then the default is NULL.

	      Note: If a default value is specified 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.

       NOT NULL
	      Values of this domain are not allowed to be NULL.

       NULL   Values of this domain are allowed to be NULL. This is the default.

	      This clause is only available for compatibility with non-standard SQL databases. Its use is discouraged in new applications.

   OUTPUTS
       CREATE DOMAIN
	      Message returned if the domain is successfully created.

DESCRIPTION
       CREATE DOMAIN allows the user to register a new data domain with PostgreSQL for use in the current data base. The user who defines a domain
       becomes its owner.

       If a schema name is given (for example, CREATE DOMAIN myschema.mydomain ...) then the domain is created in the specified schema.  Otherwise
       it  is created in the current schema (the one at the front of the search path; see CURRENT_SCHEMA()).  The domain name must be unique among
       the types and domains existing in its schema.

       Domains are useful for abstracting common fields between tables into a single location for maintenance. An email address column may be used
       in several tables, all with the same properties. Define a domain and use that rather than setting up each table's constraints individually.

EXAMPLES
       This example creates the country_code data type and then uses the type in a table definition:

       CREATE DOMAIN country_code char(2) NOT NULL;
       CREATE TABLE countrylist (id INT4, country country_code);

COMPATIBILITY
       SQL99 defines CREATE DOMAIN, but says that the only allowed constraint type is CHECK constraints. CHECK constraints for domains are not yet
       supported by PostgreSQL.

SEE ALSO
       DROP DOMAIN [drop_domain(7)], PostgreSQL Programmer's Guide

SQL - Language Statements					    2002-11-22							  CREATE DOMAIN(7)