CREATE DOMAIN(7) PostgreSQL 15.7 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 domain has the same collation behavior as its underlying data
type. 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 prevented from being null (but see notes
below).
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. 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.)
NOTES
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 restore 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.
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 15.7 2024 CREATE DOMAIN(7)
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