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 values).  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 exist-
       ing in its schema.

       Domains are useful for abstracting common constraints on fields into a single location for maintenance. For example, sev-
       eral  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                                  2011-09-22                                           CREATE DOMAIN(7)