<!--
doc/src/sgml/ref/create_domain.sgml
PostgreSQL documentation
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<refentry id="SQL-CREATEDOMAIN">
 <indexterm zone="sql-createdomain">
  <primary>CREATE DOMAIN</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE DOMAIN</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE DOMAIN</refname>
  <refpurpose>define a new domain</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE DOMAIN <replaceable class="parameter">name</replaceable> [ AS ] <replaceable class="parameter">data_type</replaceable>
    [ COLLATE <replaceable>collation</replaceable> ]
    [ DEFAULT <replaceable>expression</replaceable> ]
    [ <replaceable class="PARAMETER">constraint</replaceable> [ ... ] ]

<phrase>where <replaceable class="PARAMETER">constraint</replaceable> is:</phrase>

[ CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ]
{ NOT NULL | NULL | CHECK (<replaceable class="PARAMETER">expression</replaceable>) }
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>CREATE DOMAIN</command> 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.
  </para>

  <para>
   If a schema name is given (for example, <literal>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.
  </para>

  <para>
   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.
  </para>

  <para>
   To be able to create a domain, you must have <literal>USAGE</literal>
   privilege on the underlying type.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

    <variablelist>
     <varlistentry>
      <term><replaceable class="parameter">name</replaceable></term>
      <listitem>
       <para>
        The name (optionally schema-qualified) of a domain to be created.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">data_type</replaceable></term>
      <listitem>
       <para>
        The underlying data type of the domain. This can include array
        specifiers.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable>collation</replaceable></term>
      <listitem>
       <para>
        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 <literal>COLLATE</>
        is specified.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>DEFAULT <replaceable>expression</replaceable></literal></term>

      <listitem>
       <para>
        The <literal>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.
       </para>

       <para>
        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.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable></literal></term>
      <listitem>
       <para>
        An optional name for a constraint.  If not specified,
        the system generates a name.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>NOT NULL</></term>
      <listitem>
       <para>
        Values of this domain are normally prevented from being null.
        However, it is still possible for a domain with this constraint
        to take a null value if it is assigned a matching domain type
        that has become null, e.g. via a LEFT OUTER JOIN, or
        <command>INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM
        tab WHERE false))</command>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>NULL</></term>
      <listitem>
       <para>
        Values of this domain are allowed to be null.  This is the default.
       </para>

       <para>
        This clause is only intended for compatibility with
        nonstandard SQL databases.  Its use is discouraged in new
        applications.
       </para>
      </listitem>
     </varlistentry>

   <varlistentry>
    <term><literal>CHECK (<replaceable class="PARAMETER">expression</replaceable>)</literal></term>
    <listitem>
     <para><literal>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 <literal>VALUE</>
      to refer to the value being tested.
     </para>

     <para>
      Currently, <literal>CHECK</literal> expressions cannot contain
      subqueries nor refer to variables other than <literal>VALUE</>.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   This example creates the <type>us_postal_code</type> 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:

<programlisting>
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
);
</programlisting></para>
 </refsect1>

 <refsect1 id="SQL-CREATEDOMAIN-compatibility">
  <title>Compatibility</title>

  <para>
   The command <command>CREATE DOMAIN</command> conforms to the SQL
   standard.
  </para>
 </refsect1>

 <refsect1 id="SQL-CREATEDOMAIN-see-also">
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-alterdomain"></member>
   <member><xref linkend="sql-dropdomain"></member>
  </simplelist>
 </refsect1>

</refentry>
