CREATE DOMAIN(7)	PostgreSQL 9.2.24 Documentation	      CREATE DOMAIN(7)



NAME
       CREATE_DOMAIN - define a new domain

SYNOPSIS
       CREATE DOMAIN name [ AS ] data_type
	   [ COLLATE collation ]
	   [ 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 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.

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.

       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_name
	   An optional name for a constraint. If not specified, the system
	   generates a name.

       NOT NULL
	   Values of this domain are normally prevented from being null.
	   However, it is still possible for a domain with this constraint to
	   take a null value if it is assigned a matching domain type that has
	   become null, e.g. via a LEFT OUTER JOIN, or INSERT INTO tab
	   (domcol) VALUES ((SELECT domcol FROM tab WHERE false)).

       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))



PostgreSQL 9.2.24		  2017-11-06		      CREATE DOMAIN(7)