DEFAULT VALUES;
</programlisting>
  </para>

  <para>
   You can insert multiple rows in a single command:
<programlisting>
INSERT INTO products (product_no, name, price) VALUES
    (1, 'Cheese', 9.99),
    (2, 'Bread', 1.99),
    (3, 'Milk', 2.99);
</programlisting>
  </para>

  <para>
   It is also possible to insert the result of a query (which might be no
   rows, one row, or many rows):
<programlisting>
INSERT INTO products (product_no, name, price)
  SELECT product_no, name, price FROM new_products
    WHERE release_date = 'today';
</programlisting>
   This provides the full power of the SQL query mechanism (<xref
   linkend="queries"/>) for computing the rows to be inserted.
  </para>

  <tip>
   <para>
    When inserting a lot of data at the same time, consider using
    the <xref linkend="sql-copy"/> command.
    It is not as flexible as the <xref linkend="sql-insert"/>
    command, but is more efficient. Refer
    to <xref linkend="populate"/> for more information on improving
    bulk loading performance.
   </para>
  </tip>
 </sect1>

 <sect1 id="dml-update">
  <title>Updating Data</title>

  <indexterm zone="dml-update">
   <primary>updating</primary>
  </indexterm>

  <indexterm zone="dml-update">
   <primary>UPDATE</primary>
  </indexterm>

  <para>
   The modification of data that is already in the database is
   referred to as updating.  You can update individual rows, all the
   rows in a table, or a subset of all rows.  Each column can be
   updated separately; the other columns are not affected.
  </para>

  <para>
   To update existing rows, use the <xref linkend="sql-update"/>
   command.  This requires
   three pieces of information:
   <orderedlist spacing="compact">
    <listitem>
     <para>The name of the table and column to update</para>
    </listitem>

    <listitem>
     <para>The new value of the column</para>
    </listitem>

    <listitem>
     <para>Which row(s) to update</para>
    </listitem>
   </orderedlist>
  </para>

  <para>
   Recall from <xref linkend="ddl"/> that SQL does not, in general,
   provide a unique identifier for rows.  Therefore it is not
   always possible to directly specify which row to update.
   Instead, you specify which conditions a row must meet in order to
   be updated.  Only if you have a primary key in the table (independent of
   whether you declared it or not) can you reliably address individual rows
   by choosing a condition that matches the primary key.
   Graphical database access tools rely on this fact to allow you to
   update rows individually.
  </para>

  <para>
   For example, this command updates all products that have a price of
   5 to have a price of 10:
<programlisting>
UPDATE products SET price = 10 WHERE price = 5;
</programlisting>
    This might cause zero, one, or many rows to be updated.  It is not
    an error to attempt an update that does not match any rows.
  </para>

  <para>
   Let's look at that command in detail. First is the key word
   <literal>UPDATE</literal> followed by the table name.  As usual,
   the table name can be schema-qualified, otherwise it is looked up
   in the path.  Next is the key word <literal>SET</literal> followed
   by the column name, an equal sign, and the new column value.  The
   new column value can be any scalar expression, not just a constant.
   For example, if you want to raise the price of all products by 10%
   you could use:
<programlisting>
UPDATE products SET price = price * 1.10;
</programlisting>
   As you see, the expression for the new value can refer to the existing
   value(s) in the row.  We also left out the <literal>WHERE</literal> clause.
   If it is omitted, it means that all rows in the table are updated.
   If it is present, only those rows that match the
   <literal>WHERE</literal> condition are updated.  Note that the equals
   sign in the <literal>SET</literal> clause is an assignment while
   the one in the <literal>WHERE</literal> clause is a comparison, but
   this does not