CREATE DOMAIN domainname [AS] data_type
[ DEFAULT default_expr ]
[ constraint [ ... ] ]
where constraint is:
[ CONSTRAINT constraint_name ]
{ NOT NULL | NULL }
The name (optionally schema-qualified) of a domain to be created.
The underlying data type of the domain. This may include array specifiers. Refer to the User's Guide for further information about data types and arrays.
The DEFAULT clause specifies a default value for columns of the domain data type. The value is any variable-free expression (but subselects are not allowed). The data type of the default expression must match the data type of the domain.
The default expression will be used in any insert operation that does not specify a value for the column. If there is no default for a domain, then the default is NULL.
Note: If a default value is specified 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.
An optional name for a constraint. If not specified, the system generates a name.
Values of this domain are not allowed to be NULL.
Values of this domain are allowed to be NULL. This is the default.
This clause is only available for compatibility with non-standard SQL databases. Its use is discouraged in new applications.
CREATE DOMAIN allows the user to register a new data domain with PostgreSQL for use in the current data base. 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 one at the front of the search path; see CURRENT_SCHEMA()). The domain name must be unique among the types and domains existing in its schema.
Domains are useful for abstracting common fields between tables into a single location for maintenance. An email address column may be used in several tables, all with the same properties. Define a domain and use that rather than setting up each table's constraints individually.
This example creates the country_code data type and then uses the type in a table definition:
CREATE DOMAIN country_code char(2) NOT NULL; CREATE TABLE countrylist (id INT4, country country_code);