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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_domain.sgml,v 1.5 2002/04/23 02:07:15 tgl Exp $
PostgreSQL documentation
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<refentry id="SQL-CREATEDOMAIN">
 <refmeta>
  <refentrytitle id="sql-createdomian-title">
   CREATE DOMAIN
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   CREATE DOMAIN
  </refname>
  <refpurpose>
   define a new domain
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2002-02-24</date>
  </refsynopsisdivinfo>
  <synopsis>
CREATE DOMAIN <replaceable class="parameter">domainname</replaceable> [AS] <replaceable class="parameter">data_type</replaceable>
    [ DEFAULT <replaceable>default_expr</> ]
    [ <replaceable class="PARAMETER">constraint</replaceable> [, ... ] ]

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

[ CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ]
{ NOT NULL | NULL }
  </synopsis>

  <refsect2 id="R2-SQL-CREATEDOMAIN-1">
   <refsect2info>
    <date>2002-02-24</date>
   </refsect2info>
   <title>
    Parameters
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><replaceable class="parameter">domainname</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 may include array
	specifiers.
        Refer to the <citetitle>User's Guide</citetitle> for further
        information about data types and arrays.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>DEFAULT
      <replaceable>default_expr</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 subselects are not allowed).
	The
        data type of the default expression must match the data type of the
        domain.
       </para>

       <para>
        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.
       </para>

       <note>
        <para>
         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.
        </para>
       </note>
      </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 not allowed to be NULL.  This is
        equivalent to the column constraint <literal>CHECK (<replaceable
        class="PARAMETER">column</replaceable> NOT NULL)</literal>.
       </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 available for compatibility with
        non-standard SQL databases.  Its use is discouraged in new
        applications.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-CREATEDOMAIN-2">
   <refsect2info>
    <date>2002-02-24</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>
CREATE DOMAIN
       </computeroutput></term>
      <listitem>
       <para>
	Message returned if the domain is successfully created.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-CREATEDOMAIN-1">
  <refsect1info>
   <date>2002-02-24</date>
  </refsect1info>
  <title>
   Description
  </title>

  <para>
   <command>CREATE DOMAIN</command>  allows  the user to register a new
   data domain with <productname>PostgreSQL</> for use in the
   current data base.   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 one
   at the front of the search path; see <literal>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 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.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>
  <para>
   This example creates the <type>country_code</type> data type and then uses the
   type in a table definition:
<programlisting>
CREATE DOMAIN country_code char(2) NOT NULL;
CREATE TABLE countrylist (id INT4, country country_code);
</programlisting>
  </para>
 </refsect1>

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

  <para>
   SQL99 defines CREATE DOMAIN, but says that the only allowed constraint
   type is CHECK constraints.  CHECK constraints for domains are not yet
   supported by <productname>PostgreSQL</productname>.
  </para>
 </refsect1>

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

  <simplelist type="inline">
   <member><xref linkend="sql-dropdomain"></member>
   <member><citetitle>PostgreSQL Programmer's Guide</citetitle></member>
  </simplelist>
 </refsect1>

</refentry>


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