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- A column can be assigned a default value. When a new row is
- created and no values are specified for some of the columns, those
- 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 class="xref" href="dml.html" title="Chapter 6. Data Manipulation">Chapter 6</a>.)
- </p><p>
- <a id="id-1.5.4.4.4.1" class="indexterm"></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 class="emphasis"><strong>DEFAULT 9.99</strong></span>
- );
- </pre><p>
- </p><p>
- The default value can 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 for a <code class="type">timestamp</code> column to have a default of <code class="literal">CURRENT_TIMESTAMP</code>,
- so that it gets set to the time of row insertion. Another common
- example is generating a <span class="quote">“<span class="quote">serial number</span>”</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 class="emphasis"><strong>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 class="xref" href="functions-sequence.html" title="9.16. Sequence Manipulation Functions">Section 9.16</a>). This arrangement is sufficiently common
- that there's a special shorthand for it:
- </p><pre class="programlisting">
- CREATE TABLE products (
- product_no <span class="emphasis"><strong>SERIAL</strong></span>,
- ...
- );
- </pre><p>
- The <code class="literal">SERIAL</code> shorthand is discussed further in <a class="xref" href="datatype-numeric.html#DATATYPE-SERIAL" title="8.1.4. Serial Types">Section 8.1.4</a>.
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl-basics.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-generated-columns.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">5.1. Table Basics </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 5.3. Generated Columns</td></tr></table></div></body></html>
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