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

NAME

       CREATE DOMAIN - define a new domain

SYNOPSIS

       CREATE DOMAIN name [ AS ] data_type
	   [ DEFAULT expression ]
	   [ constraint [ ... ] ]

       where constraint is:

       [ CONSTRAINT constraint_name ]
       { NOT NULL | NULL | CHECK (expression) }

DESCRIPTION

       CREATE  DOMAIN creates a new domain. A domain is essentially a data type with optional constraints (restrictions on the allowed set of val-
       ues).  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 domain name must be unique among the 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.  Define a domain rather  than  setting
       up each table's constraint individually.

PARAMETERS

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

       data_type
	      The underlying data type of the domain. This can include array specifiers.

       DEFAULT expression
	      The  DEFAULT  clause  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.

       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 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.

EXAMPLES

       This example creates the us_postal_code data type and then uses the type in a table definition. A regular expression test is used to verify
       that the value looks like a valid US postal code:

       CREATE DOMAIN us_postal_code AS TEXT
       CHECK(
	  VALUE ~ '^\d{5}$'
       OR VALUE ~ '^\d{5}-\d{4}$'
       );

       CREATE TABLE us_snail_addy (
	 address_id SERIAL PRIMARY KEY,
	 street1 TEXT NOT NULL,
	 street2 TEXT,
	 street3 TEXT,
	 city TEXT NOT NULL,
	 postal us_postal_code NOT NULL
       );

COMPATIBILITY

       The command CREATE DOMAIN conforms to the SQL standard.

SEE ALSO

       ALTER DOMAIN [alter_domain(7)], DROP DOMAIN [drop_domain(7)]

SQL - Language Statements					    2010-05-14							  CREATE DOMAIN(7)