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CREATE DOMAIN(7)	PostgreSQL 9.6.3 Documentation	      CREATE DOMAIN(7)

       CREATE_DOMAIN - define a	new domain

       CREATE DOMAIN name [ AS ] data_type
	   [ COLLATE collation ]
	   [ DEFAULT expression	]
	   [ constraint	[ ... ]	]

       where constraint	is:

       [ CONSTRAINT constraint_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.

	   The name (optionally	schema-qualified) of a domain to be created.

	   The underlying data type of the domain. This	can include array

	   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 prevented from being null (but see	notes

	   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.)

       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.

       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
	      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.


PostgreSQL 9.6.3		     2017		      CREATE DOMAIN(7)


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