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CREATE DOMAIN(7)                 SQL Commands                 CREATE DOMAIN(7)

       CREATE DOMAIN - define a new domain

       CREATE DOMAIN name [ AS ] data_type
           [ 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.

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

              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 vari-
              able-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

              The  default  expression will be used in any insert operation that does not specify a value for the col-
              umn. 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

       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 con-
              straint 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.

       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.

       ALTER DOMAIN [alter_domain(7)], DROP DOMAIN [drop_domain(7)]

SQL - Language Statements         2014-02-17                  CREATE DOMAIN(7)