<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html> <html lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <!-- sql-programming.qdoc --> <title>Connecting to Databases | Qt SQL 5.12.2</title> <link rel="stylesheet" type="text/css" href="style/offline-simple.css" /> <script type="text/javascript"> document.getElementsByTagName("link").item(0).setAttribute("href", "style/offline.css"); // loading style sheet breaks anchors that were jumped to before // so force jumping to anchor again setTimeout(function() { var anchor = location.hash; // need to jump to different anchor first (e.g. none) location.hash = "#"; setTimeout(function() { location.hash = anchor; }, 0); }, 0); </script> </head> <body> <div class="header" id="qtdocheader"> <div class="main"> <div class="main-rounded"> <div class="navigationbar"> <table><tr> <td >Qt 5.12</td><td ><a href="qtsql-index.html">Qt SQL</a></td><td >Connecting to Databases</td></tr></table><table class="buildversion"><tr> <td id="buildversion" width="100%" align="right"><a href="qtsql-index.html">Qt 5.12.2 Reference Documentation</a></td> </tr></table> </div> </div> <div class="content"> <div class="line"> <div class="content mainContent"> <link rel="prev" href="sql-programming.html" /> <link rel="next" href="sql-sqlstatements.html" /> <p class="naviNextPrevious headerNavi"> <a class="prevPage" href="sql-programming.html">SQL Programming</a> <span class="naviSeparator"> ◦ </span> <a class="nextPage" href="sql-sqlstatements.html">Executing SQL Statements</a> </p><p/> <div class="sidebar"><div class="sidebar-content" id="sidebar-content"></div></div> <h1 class="title">Connecting to Databases</h1> <span class="subtitle"></span> <!-- $$$sql-connecting.html-description --> <div class="descr"> <a name="details"></a> <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>); bool 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 <code>flightdb</code> on the host <code>bigblue</code>. The <code>"QMYSQL"</code> 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 <code>"first"</code> and <code>"second"</code>:</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 <code>false</code>. 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> </div> <!-- @@@sql-connecting.html --> <p class="naviNextPrevious footerNavi"> <a class="prevPage" href="sql-programming.html">SQL Programming</a> <span class="naviSeparator"> ◦ </span> <a class="nextPage" href="sql-sqlstatements.html">Executing SQL Statements</a> </p> </div> </div> </div> </div> </div> <div class="footer"> <p> <acronym title="Copyright">©</acronym> 2019 The Qt Company Ltd. 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