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<h1>8. SQL Reference</h1>
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<h2>8.16. Sequence Objects</h2>
<p>
Virtuoso supports sequence objects. These can be used to generate sequential
numbers which can be used as unique identifiers. A sequence object is
guaranteed never to give the same number twice. Each sequence has a
name and a state. The state of a sequence is stored in the database at
checkpoint time. Between checkpoints sequence states are logged so that
a possible roll forward recovery will not lose information.
</p>
<p>
The SQL functions <span class="computeroutput">sequence_next</span>() and <span class="computeroutput">sequence_set</span>() are used to access
and set the state of sequences. These take the name of the sequence as
argument. This is a server/wide unique string. There are no restrictions
on the length or character set of the sequence
</p>
<p>
Sequences do not have to be separately created. A sequence object
will automatically be generated when first referenced by <span class="computeroutput">sequence_next</span>()
or sequence_set.
</p>
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<pre class="programlisting">sequence_next (in name varchar) returns integer
</pre>
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<pre class="programlisting">sequence_set (in name varchar, in state integer, in mode integer)
returns integer
</pre>
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<p>
Function <span class="computeroutput">sequence_next</span>() returns the current state of the specified sequence and
atomically increments it by one. The next call will thus return a
number one greater than the previous. The sequence is shared between
all connections and all transactions. Using a sequence never involves
locking.
</p>
<p>
Function <span class="computeroutput">sequence_set</span>() sets and returns the state of a sequence object. The next
call to <span class="computeroutput">sequence_next</span>() will return this same number. If mode equals 0,
the state is set regardless of the previous state. If mode is non-zero,
the state is set only if the new state is greater then the previous
state. Calling sequence_set ('sequence'' , 0, 1) will always return
the sequence's state without changing it.
</p>
<p>
Each autoincrement column corresponds to an internal sequence object.
The name of the sequence object is'DB.DBA.' plus the concatenation
of the table's qualifier, owner, table name and column name,
e.g. 'DB.DBA.db.dba.my_table.ai_column'. The user does not
normally need to know about the sequence associated with an autoincrement
column unless he or she wishes to change the sequence values using the
<span class="computeroutput">sequence_set()</span> function and the sequence objects name.</p>
<p>
See the section on identity columns under create table and the function identity_value and the related ODBC statement option SQL_GETLASTSERIAL for more.
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