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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>GNOME Data Access 5 manual: Virtual connections</title>
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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="virtual_connection"></a>Virtual connections</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt>
<span class="refentrytitle"><a href="GdaVirtualProvider.html">GdaVirtualProvider</a></span><span class="refpurpose"> — Base class for all virtual provider objects</span>
</dt>
<dt>
<span class="refentrytitle"><a href="GdaVproviderDataModel.html">GdaVproviderDataModel</a></span><span class="refpurpose"> — Virtual provider for connections based on a list of GdaDataModel</span>
</dt>
<dt>
<span class="refentrytitle"><a href="GdaVproviderHub.html">GdaVproviderHub</a></span><span class="refpurpose"> — Virtual provider for connections based on other connection</span>
</dt>
<dt>
<span class="refentrytitle"><a href="GdaVirtualConnection.html">GdaVirtualConnection</a></span><span class="refpurpose"> — Base class for all virtual connection objects</span>
</dt>
<dt>
<span class="refentrytitle"><a href="GdaVconnectionDataModel.html">GdaVconnectionDataModel</a></span><span class="refpurpose"> — Virtual connection based on a list of GdaDataModel</span>
</dt>
<dt>
<span class="refentrytitle"><a href="GdaVconnectionHub.html">GdaVconnectionHub</a></span><span class="refpurpose"> — Virtual connection which bind together connections</span>
</dt>
<dt>
<span class="refentrytitle"><a href="libgda-5.0-GdaLdapConnection.html">GdaLdapConnection</a></span><span class="refpurpose"> — LDAP connection objects</span>
</dt>
</dl></div>
<div class="sect2">
<div class="titlepage"><div><div><h3 class="title">
<a name="VirtualIntro"></a>Introduction to virtual connections</h3></div></div></div>
<p>
    <span class="application">Libgda</span> implements so called virtual connections which are used like normal 
    <a class="link" href="GdaConnection.html" title="GdaConnection">GdaConnection</a> objects (open, close, execute SQL commands, etc)
    but which in fact don't access any real DBMS.
  </p>
<p>
    Virtual connections allow one to use SQL to work on data when it is normally not available, such as sorting data
    physically stored in CSV or XML files.
  </p>
<p>
    A virtual connection is not created in the same way as for a normal connection:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>A specific <a class="link" href="GdaServerProvider.html" title="GdaServerProvider">GdaServerProvider</a> object is required</p></li>
<li class="listitem"><p>No data source (DSN) or connection string can be used</p></li>
<li class="listitem"><p>One must use the <a class="link" href="GdaVirtualConnection.html#gda-virtual-connection-open" title="gda_virtual_connection_open ()">gda_virtual_connection_open()</a>
	  method which creates and "opens" a virtual connection. </p></li>
</ul></div>
<p>
    Here is an example of code on how to create a virtual connection using the <a class="link" href="GdaVproviderDataModel.html" title="GdaVproviderDataModel">GdaVproviderDataModel</a> virtual provider:
    </p>
<pre class="programlisting">
GdaConnection *cnc;
GdaVirtualProvider *provider;

provider = gda_vprovider_data_model_new ();
cnc = gda_virtual_connection_open (provider, NULL);
    </pre>
<p>
  </p>
<p>
    Some examples of virtual connections usage can be found in the source distribution of Libgda in:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p><code class="filename">samples/Virtual</code></p></li>
<li class="listitem"><p><code class="filename">samples/DirDataModel</code></p></li>
<li class="listitem"><p><code class="filename">samples/F-Spot</code></p></li>
<li class="listitem"><p><code class="filename">testing/virtual-test.c</code></p></li>
<li class="listitem"><p><code class="filename">testing/virtual-test-2.c</code></p></li>
</ul></div>
<p>
  </p>
<p>Note that virtual connections have some inherent limitations
  <a href="#ftn.id-1.3.7.2.6.1" class="footnote" name="id-1.3.7.2.6.1"><sup class="footnote">[1]</sup></a>due to the implementation.
</p>
</div>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.id-1.3.7.2.6.1" class="footnote">
<p><a href="#id-1.3.7.2.6.1" class="para"><sup class="para">[1] </sup></a>
      As virtual connections are implemented using 
      <a class="ulink" href="http://www.sqlite.org/cvstrac/wiki?p=VirtualTables" target="_top">SQLite's virtual table</a> features, 
      the SQL dialect which can be used
      is the SQLite one (see the <a class="ulink" href="http://sqlite.org/lang.html" target="_top">SQL as Understood By SQLite</a> page), and
      there are a few limitations inherent to this implementation (see link for more information).
    </p>
<p>
      Also note that it is possible to create temporary tables in virtual connections using "CREATE TEMP TABLE..." statements,
      but the contents of such tables will be lost once the connection is closed.
    </p>
</div>
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