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>6.2. Updating Data</A
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>   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.
  </P
><P
>   To perform an update, you need three pieces of information:
   <P
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>The name of the table and column to update,</P
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>The new value of the column,</P
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>Which row(s) to update.</P
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  </P
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>   Recall from <A
HREF="ddl.html"
>Chapter 5</A
> that SQL does not, in general,
   provide a unique identifier for rows.  Therefore it is not
   necessarily 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 (no matter
   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.
  </P
><P
>   For example, this command updates all products that have a price of
   5 to have a price of 10:
</P><PRE
CLASS="PROGRAMLISTING"
>UPDATE products SET price = 10 WHERE price = 5;</PRE
><P>
    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.
  </P
><P
>   Let's look at that command in detail. First is the key word
   <TT
CLASS="LITERAL"
>UPDATE</TT
> 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 <TT
CLASS="LITERAL"
>SET</TT
> followed
   by the column name, an equals 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:
</P><PRE
CLASS="PROGRAMLISTING"
>UPDATE products SET price = price * 1.10;</PRE
><P>
   As you see, the expression for the new value can refer to the existing
   value(s) in the row.  We also left out the <TT
CLASS="LITERAL"
>WHERE</TT
> 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
   <TT
CLASS="LITERAL"
>WHERE</TT
> condition are updated.  Note that the equals
   sign in the <TT
CLASS="LITERAL"
>SET</TT
> clause is an assignment while
   the one in the <TT
CLASS="LITERAL"
>WHERE</TT
> clause is a comparison, but
   this does not create any ambiguity.  Of course, the
   <TT
CLASS="LITERAL"
>WHERE</TT
> condition does
   not have to be an equality test.  Many other operators are
   available (see <A
HREF="functions.html"
>Chapter 9</A
>).  But the expression
   needs to evaluate to a Boolean result.
  </P
><P
>   You can update more than one column in an
   <TT
CLASS="COMMAND"
>UPDATE</TT
> command by listing more than one
   assignment in the <TT
CLASS="LITERAL"
>SET</TT
> clause.  For example:
</P><PRE
CLASS="PROGRAMLISTING"
>UPDATE mytable SET a = 5, b = 3, c = 1 WHERE a &gt; 0;</PRE
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