postgres/doc/src/sgml/ref/create_domain.sgml

<!--
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">domain_constraint</replaceable> [ ... ] ]

<phrase>where <replaceable class="parameter">domain_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 ...</literal>) 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 domain has the same collation behavior as its
        underlying data type.
        The underlying type must be collatable if <literal>COLLATE</literal>
        is specified.
       </para>
      </listitem>
     </varlistentry>

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

      <listitem>
       <para>
        The <literal>DEFAULT</literal> 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</literal></term>
      <listitem>
       <para>
        Values of this domain are prevented from being null
        (but see notes below).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>NULL</literal></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</literal> 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</literal>
      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.
     </para>

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

     <para>
      When a domain has multiple <literal>CHECK</literal> constraints,
      they will be tested in alphabetical order by name.
      (<productname>PostgreSQL</productname> versions before 9.5 did not honor any
      particular firing order for <literal>CHECK</literal> constraints.)
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1 id="sql-createdomain-notes">
  <title>Notes</title>

  <para>
   Domain constraints, particularly <literal>NOT NULL</literal>, 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
<programlisting>
INSERT INTO tab (domcol) VALUES ((SELECT domcol FROM tab WHERE false));
</programlisting>
   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.
  </para>

  <para>
   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 <literal>NOT NULL</literal> constraints to columns of
   the domain type as needed, rather than directly to the domain type.
  </para>

  <para>
   <productname>PostgreSQL</productname> assumes that
   <literal>CHECK</literal> constraints' conditions are immutable, that is,
   they will always give the same result for the same input value.  This
   assumption is what justifies examining <literal>CHECK</literal>
   constraints only when a value is first converted to be of a domain type,
   and not at other times.  (This is essentially the same as the treatment
   of table <literal>CHECK</literal> constraints, as described in
   <xref linkend="ddl-constraints-check-constraints"/>.)
  </para>

  <para>
   An example of a common way to break this assumption is to reference a
   user-defined function in a <literal>CHECK</literal> expression, and then
   change the behavior of that
   function.  <productname>PostgreSQL</productname> does not disallow that,
   but it will not notice if there are stored values of the domain type that
   now violate the <literal>CHECK</literal> constraint. That would cause a
   subsequent database dump and restore to fail.  The recommended way to
   handle such a change is to drop the constraint (using <command>ALTER
   DOMAIN</command>), adjust the function definition, and re-add the
   constraint, thereby rechecking it against stored data.
  </para>

  <para>
   It's also good practice to ensure that domain <literal>CHECK</literal>
   expressions will not throw errors.
  </para>
 </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>

  <para>
   The syntax <literal>NOT NULL</literal> in this command is a
   <productname>PostgreSQL</productname> extension.  (A standard-conforming
   way to write the same would be <literal>CHECK (VALUE IS NOT
   NULL)</literal>.  However, per <xref linkend="sql-createdomain-notes"/>,
   such constraints are best avoided in practice anyway.)  The
   <literal>NULL</literal> <quote>constraint</quote> is a
   <productname>PostgreSQL</productname> extension (see also <xref
   linkend="sql-createtable-compatibility"/>).
  </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>