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<title>Tracker SPARQL Library Reference Manual: Querying the Store</title>
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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="tracker-examples-readonly"></a>Querying the Store</h2></div></div></div>
<p>
In order to perform read-only queries to the store, a new
<span class="type"><a class="link" href="TrackerSparqlConnection.html#TrackerSparqlConnection-struct" title="struct TrackerSparqlConnection">TrackerSparqlConnection</a></span>
object must be acquired. In this case, as there is no intention of updating any
value in the store, both the general connection (with
<code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-get" title="tracker_sparql_connection_get ()">tracker_sparql_connection_get</a></code>)
or a specific direct-access connection (with
<code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-get-direct" title="tracker_sparql_connection_get_direct ()">tracker_sparql_connection_get_direct</a></code>)
may be acquired. Note that in the latter case, every non read-only operation will result
in an error being thrown by the TrackerSparqlConnection.
</p>
<p>
Once a proper connection object has been acquired, the read-only query can be launched either
synchronously (<code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-query" title="tracker_sparql_connection_query ()">tracker_sparql_connection_query</a></code>)
or asynchronously (<code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-query-async" title="tracker_sparql_connection_query_async ()">tracker_sparql_connection_query_async</a></code>).
If launched asynchronously, the results of the query can be obtained with
<code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-query-finish" title="tracker_sparql_connection_query_finish ()">tracker_sparql_connection_query_finish</a></code>.
</p>
<p>
If the query was successful, a <span class="type"><a class="link" href="TrackerSparqlCursor.html#TrackerSparqlCursor-struct" title="struct TrackerSparqlCursor">TrackerSparqlCursor</a></span>
will be available. You can now iterate the results of the query both synchronously (with
<code class="function"><a class="link" href="TrackerSparqlCursor.html#tracker-sparql-cursor-next" title="tracker_sparql_cursor_next ()">tracker_sparql_cursor_next</a></code>) or
asynchronously (with
<code class="function"><a class="link" href="TrackerSparqlCursor.html#tracker-sparql-cursor-next-async" title="tracker_sparql_cursor_next_async ()">tracker_sparql_cursor_next_async</a></code> and
<code class="function"><a class="link" href="TrackerSparqlCursor.html#tracker-sparql-cursor-next-finish" title="tracker_sparql_cursor_next_finish ()">tracker_sparql_cursor_next_finish</a></code>)
</p>
<p>
Once you end up with the query, remember to call <code class="function"><a href="/usr/share/gtk-doc/html/gobject/gobject-The-Base-Object-Type.html#g-object-unref">g_object_unref</a></code>
for the <span class="type"><a class="link" href="TrackerSparqlCursor.html#TrackerSparqlCursor-struct" title="struct TrackerSparqlCursor">TrackerSparqlCursor</a></span>. And the same applies to the
<span class="type"><a class="link" href="TrackerSparqlConnection.html#TrackerSparqlConnection-struct" title="struct TrackerSparqlConnection">TrackerSparqlConnection</a></span> when no longer needed.
</p>
<p>
The following program shows how Read-Only queries can be done to the store in a
synchronous way:
</p>
<pre class="programlisting">
#include <tracker-sparql.h>
int main (int argc, const char **argv)
{
GError *error = NULL;
<span class="type"><a class="link" href="TrackerSparqlConnection.html#TrackerSparqlConnection-struct" title="struct TrackerSparqlConnection">TrackerSparqlConnection</a></span> *connection;
<span class="type"><a class="link" href="TrackerSparqlCursor.html#TrackerSparqlCursor-struct" title="struct TrackerSparqlCursor">TrackerSparqlCursor</a></span> *cursor;
const gchar *query = "SELECT nie:url(?u) WHERE { ?u a nfo:FileDataObject }";
/* As we know only read-only queries will be done, it's enough
* to use a connection with only direct-access setup. The NULL
* represents a possible GCancellable.
*/
connection = <code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-get-direct" title="tracker_sparql_connection_get_direct ()">tracker_sparql_connection_get_direct</a></code> (NULL, &error);
if (!connection) {
g_printerr ("Couldn't obtain a direct connection to the Tracker store: %s",
error ? error->message : "unknown error");
g_clear_error (&error);
return 1;
}
/* Make a synchronous query to the store */
cursor = <code class="function"><a class="link" href="TrackerSparqlConnection.html#tracker-sparql-connection-query" title="tracker_sparql_connection_query ()">tracker_sparql_connection_query</a></code> (connection,
query,
NULL,
&error);
if (error) {
/* Some error happened performing the query, not good */
g_printerr ("Couldn't query the Tracker Store: '%s'",
error ? error->message : "unknown error");
g_clear_error (&error);
return 1;
}
/* Check results... */
if (!cursor) {
g_print ("No results found :-/\n");
} else {
gint i = 0;
/* Iterate, synchronously, the results... */
while (<code class="function"><a class="link" href="TrackerSparqlCursor.html#tracker-sparql-cursor-next" title="tracker_sparql_cursor_next ()">tracker_sparql_cursor_next</a></code> (cursor, NULL, &error)) {
g_print ("Result [%d]: %s\n",
i++,
<code class="function"><a class="link" href="TrackerSparqlCursor.html#tracker-sparql-cursor-get-string" title="tracker_sparql_cursor_get_string ()">tracker_sparql_cursor_get_string</a></code> (cursor, 0, NULL));
}
g_print ("A total of '%d' results were found\n", i);
g_object_unref (cursor);
}
g_object_unref (connection);
return 0;
}
</pre>
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