column, then the default is null.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-createtable-parms-generated-stored">
    <term><literal>GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) [ STORED | VIRTUAL ]</literal><indexterm><primary>generated column</primary></indexterm></term>
    <listitem>
     <para>
      This clause creates the column as a <firstterm>generated
      column</firstterm>.  The column cannot be written to, and when read the
      result of the specified expression will be returned.
     </para>

     <para>
      When <literal>VIRTUAL</literal> is specified, the column will be
      computed when it is read, and it will not occupy any storage.  When
      <literal>STORED</literal> is specified, the column will be computed on
      write and will be stored on disk.  <literal>VIRTUAL</literal> is the
      default.
     </para>

     <para>
      The generation expression can refer to other columns in the table, but
      not other generated columns.  Any functions and operators used must be
      immutable.  References to other tables are not allowed.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-createtable-parms-generated-identity">
    <term><literal>GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ]</literal></term>
    <listitem>
     <para>
      This clause creates the column as an <firstterm>identity
      column</firstterm>.  It will have an implicit sequence attached to it
      and in newly-inserted rows the column will automatically have values
      from the sequence assigned to it.
      Such a column is implicitly <literal>NOT NULL</literal>.
     </para>

     <para>
      The clauses <literal>ALWAYS</literal> and <literal>BY DEFAULT</literal>
      determine how explicitly user-specified values are handled in
      <command>INSERT</command> and <command>UPDATE</command> commands.
     </para>

     <para>
      In an <command>INSERT</command> command, if <literal>ALWAYS</literal> is
      selected, a user-specified value is only accepted if the
      <command>INSERT</command> statement specifies <literal>OVERRIDING SYSTEM
      VALUE</literal>.  If <literal>BY DEFAULT</literal> is selected, then the
      user-specified value takes precedence.  See <xref linkend="sql-insert"/>
      for details.  (In the <command>COPY</command> command, user-specified
      values are always used regardless of this setting.)
     </para>

     <para>
      In an <command>UPDATE</command> command, if <literal>ALWAYS</literal> is
      selected, any update of the column to any value other than
      <literal>DEFAULT</literal> will be rejected.  If <literal>BY
      DEFAULT</literal> is selected, the column can be updated normally.
      (There is no <literal>OVERRIDING</literal> clause for the
      <command>UPDATE</command> command.)
     </para>

     <para>
      The optional <replaceable>sequence_options</replaceable> clause can
      be used to override the parameters of the sequence.  The available
      options include those shown for <xref linkend="sql-createsequence"/>,
      plus <literal>SEQUENCE NAME <replaceable>name</replaceable></literal>,
      <literal>LOGGED</literal>, and <literal>UNLOGGED</literal>, which
      allow selection of the name and persistence level of the
      sequence.  Without <literal>SEQUENCE NAME</literal>, the system
      chooses an unused name for the sequence.
      Without <literal>LOGGED</literal> or <literal>UNLOGGED</literal>,
      the sequence will have the same persistence level as the table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-createtable-parms-unique">
    <term><literal>UNIQUE [ NULLS [ NOT ] DISTINCT ]</literal> (column constraint)</term>
    <term><literal>UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] [, <replaceable class="parameter">column_name</replaceable>