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       CREATE DOMAIN - define a new domain


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

       where constraint is:

       [ 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.

       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.


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

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

       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

       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

              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.


       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
          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


       The command CREATE DOMAIN conforms to the SQL standard.


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