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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<title>Using Sequences</title>
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<p>Library Version 11.2.5.3</p>
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<table width="100%" summary="Navigation header">
<tr>
<th colspan="3" align="center">Using Sequences</th>
</tr>
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<div class="sect1" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both"><a id="sequencesupport"></a>Using Sequences</h2>
</div>
</div>
</div>
<div class="toc">
<dl>
<dt>
<span class="sect2">
<a href="sequencesupport.html#create_sequence">create_sequence</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="sequencesupport.html#seq_nextval">nextval</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="sequencesupport.html#seq_currval">currval</a>
</span>
</dt>
<dt>
<span class="sect2">
<a href="sequencesupport.html#seq_drop_sequence">drop_sequence</a>
</span>
</dt>
</dl>
</div>
<p>
You can use sequences with the SQL API.
Sequences provide for an arbitrary number of increasing or
decreasing integers that persist across database accesses.
Use sequences if you need to create unique values in a
highly efficient and persistent way.
</p>
<p>
To create and access a sequence, you must use SQL
functionality that is unique to the BDB SQL interface; no
corresponding functionality exists in SQLite. The
sequence functionality is implemented using SQLite
function plugins, as such it is necessary to use the
'select' keyword as a prefix to all sequence APIs.
</p>
<p>
The SQL API sequence support is a partial implementation of
the sequence API defined in the SQL 2003 specification.
</p>
<p>
The following sections describe the BDB SQL interface sequence API.
</p>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="create_sequence"></a>create_sequence</h3>
</div>
</div>
</div>
<p>
Creates a new sequence. A name is required, all other
parameters are optional. For example:
</p>
<pre class="programlisting">SELECT create_sequence("my_sequence", "start", 100, "incr", 10,
"maxvalue", 300);</pre>
<p>
This creates a sequence called <code class="literal">my_sequence</code>
starting at 100 and incrementing by 10 until it reaches
300.
</p>
<pre class="programlisting">SELECT create_sequence("my_decr_sequence", "incr", -100,
"minvalue", -10000);</pre>
<p>
This creates a sequence call <code class="literal">my_decr_sequence</code>
starting at 0 and decreasing by 100 until it reaches
-10000.
</p>
<p>
Parameters are:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
<code class="literal">name</code>
</p>
<p>
Required parameter that provides the name of
the sequence. It is an error to create a
sequence with another name that is currently in
use within the database.
</p>
</li>
<li>
<p>
<code class="literal">start</code>
</p>
<p>
The starting value for the sequence. If this
parameter is not provided then
<code class="literal">0</code> is used.
</p>
</li>
<li>
<p>
<code class="literal">minvalue</code>
</p>
<p>
The lowest value generated by the sequence. If
this parameter is not provided and a
decrementing sequence is created, then
INT64_MIN is used.
</p>
</li>
<li>
<p>
<code class="literal">maxvalue</code>
</p>
<p>
The largest value generated by the sequence.
If this parameter is not provided and an
incrementing sequence is created, then
INT64_MAX is used.
</p>
</li>
<li>
<p>
<code class="literal">incr</code>
</p>
<p>
The amount the sequence is incremented for each
get operation. This value can be positive or
negative. If this parameter is not provided,
then <code class="literal">1</code> is used.
</p>
</li>
<li>
<p>
<code class="literal">cache</code>
</p>
<p>
Causes each handle to keep a cache of sequence
values. So long as there are values available
in the cache, retrieving the next value is
cheap and does not lead to contention between
handles.
</p>
<p>
Sequences with caches cannot be created or dropped
within an explicit transaction.
</p>
<p>
Operations on caching sequences are not
transactionally protected. That is, a rollback
will not result in a value being returned to
the sequence.
</p>
<p>
Sequences with caches do not support the
<code class="literal">currval</code> function.
</p>
<p>
The parameter following the cache parameter must be
an integer value specifying the size of the cache.
</p>
</li>
</ul>
</div>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="seq_nextval"></a>nextval</h3>
</div>
</div>
</div>
<p>
Retrieves the next value from the named sequence. For
example:
</p>
<pre class="programlisting">SELECT nextval("my_sequence");</pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="seq_currval"></a>currval</h3>
</div>
</div>
</div>
<p>
Retrieves the last value that was returned from the
named sequence. For example:
</p>
<pre class="programlisting">SELECT currval("my_sequence");</pre>
</div>
<div class="sect2" lang="en" xml:lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a id="seq_drop_sequence"></a>drop_sequence</h3>
</div>
</div>
</div>
<p>
Removes the sequence. For example:
</p>
<pre class="programlisting">SELECT drop_sequence("my_sequence");</pre>
</div>
</div>
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