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<title>6.2.Updating Data</title>
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<a name="dml-update"></a>6.2.Updating Data</h2></div></div></div>
<a name="id579701"></a><a name="id579712"></a><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 perform an update, you need three pieces of information:
</p>
<div class="orderedlist"><ol type="1" compact>
<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></div>
<p>
</p>
<p> Recall from <a href="ddl.html" title="Chapter5.Data Definition">Chapter5, <i>Data Definition</i></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 may 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
<code class="literal">UPDATE</code> followed by the table name. As usual,
the table name may be schema-qualified, otherwise it is looked up
in the path. Next is the key word <code class="literal">SET</code> 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 <code class="literal">WHERE</code> 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
<code class="literal">WHERE</code> condition are updated. Note that the equals
sign in the <code class="literal">SET</code> clause is an assignment while
the one in the <code class="literal">WHERE</code> clause is a comparison, but
this does not create any ambiguity. Of course, the
<code class="literal">WHERE</code> condition does
not have to be an equality test. Many other operators are
available (see <a href="functions.html" title="Chapter9.Functions and Operators">Chapter9, <i>Functions and Operators</i></a>). But the expression
needs to evaluate to a Boolean result.
</p>
<p> You can update more than one column in an
<code class="command">UPDATE</code> command by listing more than one
assignment in the <code class="literal">SET</code> clause. For example:
</p>
<pre class="programlisting">UPDATE mytable SET a = 5, b = 3, c = 1 WHERE a > 0;</pre>
<p>
</p>
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