CREATE DOMAIN — define a new domain
CREATE DOMAINname[ AS ]data_type[ COLLATEcollation] [ DEFAULTexpression] [constraint[ ... ] ] whereconstraintis: [ CONSTRAINTconstraint_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.
   To be able to create a domain, you must have USAGE
   privilege on the underlying type.
  
nameThe name (optionally schema-qualified) of a domain to be created.
data_typeThe underlying data type of the domain. This can include array specifiers.
collation
        An optional collation for the domain.  If no collation is
        specified, the underlying data type's default collation is used.
        The underlying type must be collatable if COLLATE
        is specified.
       
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_nameAn optional name for a constraint. If not specified, the system generates a name.
NOT NULLValues of this domain are prevented from being null (but see notes below).
NULLValues 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.  Expressions evaluating
      to TRUE or UNKNOWN succeed.  If the expression produces a FALSE result,
      an error is reported and the value is not allowed to be converted
      to the domain type.
     
      Currently, CHECK expressions cannot contain
      subqueries nor refer to variables other than VALUE.
     
      When a domain has multiple CHECK constraints,
      they will be tested in alphabetical order by name.
      (PostgreSQL versions before 9.5 did not honor any
      particular firing order for CHECK constraints.)
     
   Domain constraints, particularly NOT NULL, are checked when
   converting a value to the domain type.  It is possible for a column that
   is nominally of the domain type to read as null despite there being such
   a constraint.  For example, this can happen in an outer-join query, if
   the domain column is on the nullable side of the outer join.  A more
   subtle example is
INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false));
The empty scalar sub-SELECT will produce a null value that is considered to be of the domain type, so no further constraint checking is applied to it, and the insertion will succeed.
   It is very difficult to avoid such problems, because of SQL's general
   assumption that a null value is a valid value of every data type.  Best practice
   therefore is to design a domain's constraints so that a null value is allowed,
   and then to apply column NOT NULL constraints to columns of
   the domain type as needed, rather than directly to the domain type.
  
   PostgreSQL assumes that
   CHECK constraints' conditions are immutable, that is,
   they will always give the same result for the same input value.  This
   assumption is what justifies examining CHECK
   constraints only when a value is first converted to be of a domain type,
   and not at other times.  (This is essentially the same as the treatment
   of table CHECK constraints, as described in
   Section 5.4.1.)
  
   An example of a common way to break this assumption is to reference a
   user-defined function in a CHECK expression, and then
   change the behavior of that
   function.  PostgreSQL does not disallow that,
   but it will not notice if there are stored values of the domain type that
   now violate the CHECK constraint. That would cause a
   subsequent database dump and reload to fail.  The recommended way to
   handle such a change is to drop the constraint (using ALTER
   DOMAIN), adjust the function definition, and re-add the
   constraint, thereby rechecking it against stored data.
  
   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
);
   The command CREATE DOMAIN conforms to the SQL
   standard.