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<h1 class="title">Connecting to Databases</h1>
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<p>To access a database with <a href="qsqlquery.html">QSqlQuery</a> or <a href="qsqlquerymodel.html">QSqlQueryModel</a>, create and open one or more database connections. Database connections are normally identified by connection name, <i>not</i> by database name. You can have multiple connections to the same database. <a href="qsqldatabase.html">QSqlDatabase</a> also supports the concept of a <i>default</i> connection, which is an unnamed connection. When calling <a href="qsqlquery.html">QSqlQuery</a> or <a href="qsqlquerymodel.html">QSqlQueryModel</a> member functions that take a connection name argument, if you don't pass a connection name, the default connection will be used. Creating a default connection is convenient when your application only requires one database connection.</p>
<p>Note the difference between creating a connection and opening it. Creating a connection involves creating an instance of class <a href="qsqldatabase.html">QSqlDatabase</a>. The connection is not usable until it is opened. The following snippet shows how to create a <i>default</i> connection and then open it:</p>
<pre class="cpp"> <span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span> db <span class="operator">=</span> <span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span><span class="operator">::</span>addDatabase(<span class="string">"QMYSQL"</span>);
db<span class="operator">.</span>setHostName(<span class="string">"bigblue"</span>);
db<span class="operator">.</span>setDatabaseName(<span class="string">"flightdb"</span>);
db<span class="operator">.</span>setUserName(<span class="string">"acarlson"</span>);
db<span class="operator">.</span>setPassword(<span class="string">"1uTbSbAs"</span>);
<span class="type">bool</span> ok <span class="operator">=</span> db<span class="operator">.</span>open();</pre>
<p>The first line creates the connection object, and the last line opens it for use. In between, we initialize some connection information, including the <a href="qsqldatabase.html#setDatabaseName">database name</a>, the <a href="qsqldatabase.html#setHostName">host name</a>, the <a href="qsqldatabase.html#setUserName">user name</a>, and the <a href="qsqldatabase.html#setPassword">password</a>. In this case, we are connecting to the MySQL database <tt>flightdb</tt> on the host <tt>bigblue</tt>. The <tt>"QMYSQL"</tt> argument to <a href="qsqldatabase.html#addDatabase">addDatabase()</a> specifies the type of database driver to use for the connection. The set of database drivers included with Qt are shown in the table of <a href="sql-driver.html#supported-databases">supported database drivers</a>.</p>
<p>The connection in the snippet will be the <i>default</i> connection, because we don't pass the second argument to <a href="qsqldatabase.html#addDatabase">addDatabase()</a>, which is the connection name. For example, here we establish two MySQL database connections named <tt>"first"</tt> and <tt>"second"</tt>:</p>
<pre class="cpp"> <span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span> firstDB <span class="operator">=</span> <span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span><span class="operator">::</span>addDatabase(<span class="string">"QMYSQL"</span><span class="operator">,</span> <span class="string">"first"</span>);
<span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span> secondDB <span class="operator">=</span> <span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span><span class="operator">::</span>addDatabase(<span class="string">"QMYSQL"</span><span class="operator">,</span> <span class="string">"second"</span>);</pre>
<p>After these connections have been initialized, <a href="qsqldatabase.html#open">open()</a> for each one to establish the live connections. If the <a href="qsqldatabase.html#open">open()</a> fails, it returns false. In that case, call <a href="qsqldatabase.html#lastError">QSqlDatabase::lastError</a>() to get error information.</p>
<p>Once a connection is established, we can call the static function <a href="qsqldatabase.html#database">QSqlDatabase::database</a>() from anywhere with a connection name to get a pointer to that database connection. If we don't pass a connection name, it will return the default connection. For example:</p>
<pre class="cpp"> <span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span> defaultDB <span class="operator">=</span> <span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span><span class="operator">::</span>database();
<span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span> firstDB <span class="operator">=</span> <span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span><span class="operator">::</span>database(<span class="string">"first"</span>);
<span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span> secondDB <span class="operator">=</span> <span class="type"><a href="qsqldatabase.html">QSqlDatabase</a></span><span class="operator">::</span>database(<span class="string">"second"</span>);</pre>
<p>To remove a database connection, first close the database using <a href="qsqldatabase.html#close">QSqlDatabase::close</a>(), then remove it using the static method <a href="qsqldatabase.html#removeDatabase">QSqlDatabase::removeDatabase</a>().</p>
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