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 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)]