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><A
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>6.2. Updating Data</A
></H1
><P
> 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 update existing rows, use the <A
HREF="sql-update.html"
>UPDATE</A
>
command. This requires
three pieces of information:
<P
></P
></P><OL
COMPACT="COMPACT"
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><LI
><P
>The name of the table and column to update</P
></LI
><LI
><P
>The new value of the column</P
></LI
><LI
><P
>Which row(s) to update</P
></LI
></OL
><P>
</P
><P
> 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
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.
</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 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:
</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 > 0;</PRE
><P>
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