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<title>5.2.Default Values</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="ddl-default"></a>5.2.Default Values</h2></div></div></div>
<a name="id574642"></a><p> A column can be assigned a default value. When a new row is
created and no values are specified for some of the columns, the
columns will be filled with their respective default values. A
data manipulation command can also request explicitly that a column
be set to its default value, without having to know what that value is.
(Details about data manipulation commands are in <a href="dml.html" title="Chapter6.Data Manipulation">Chapter6, <i>Data Manipulation</i></a>.)
</p>
<p> <a name="id574671"></a>
If no default value is declared explicitly, the default value is the
null value. This usually makes sense because a null value can
be considered to represent unknown data.
</p>
<p> In a table definition, default values are listed after the column
data type. For example:
</p>
<pre class="programlisting">CREATE TABLE products (
product_no integer,
name text,
price numeric <span><strong class="emphasis">DEFAULT 9.99</strong></span>
);</pre>
<p>
</p>
<p> The default value may be an expression, which will be
evaluated whenever the default value is inserted
(<span class="emphasis"><em>not</em></span> when the table is created). A common example
is that a <code class="type">timestamp</code> column may have a default of <code class="literal">now()</code>,
so that it gets set to the time of row insertion. Another common
example is generating a “<span class="quote">serial number</span>” for each row.
In <span class="productname">PostgreSQL</span> this is typically done by
something like
</p>
<pre class="programlisting">CREATE TABLE products (
product_no integer <span><strong class="emphasis">DEFAULT nextval('products_product_no_seq')</strong></span>,
...
);</pre>
<p>
where the <code class="literal">nextval()</code> function supplies successive values
from a <em class="firstterm">sequence object</em> (see <a href="functions-sequence.html" title="9.12.Sequence Manipulation Functions">Section9.12, “Sequence Manipulation Functions”</a>). This arrangement is sufficiently common
that there's a special shorthand for it:
</p>
<pre class="programlisting">CREATE TABLE products (
product_no <span><strong class="emphasis">SERIAL</strong></span>,
...
);</pre>
<p>
The <code class="literal">SERIAL</code> shorthand is discussed further in <a href="datatype.html#datatype-serial" title="8.1.4.Serial Types">Section8.1.4, “Serial Types”</a>.
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