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SQL Programming Executing SQL Statements

# Connecting to Databases

To access a database with QSqlQuery or QSqlQueryModel, create and open one or more database connections. Database connections are normally identified by connection name, not by database name. You can have multiple connections to the same database. QSqlDatabase also supports the concept of a default connection, which is an unnamed connection. When calling QSqlQuery or QSqlQueryModel 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.

Note the difference between creating a connection and opening it. Creating a connection involves creating an instance of class QSqlDatabase. The connection is not usable until it is opened. The following snippet shows how to create a default connection and then open it:

         QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
         db.setHostName("bigblue");
         db.setDatabaseName("flightdb");
         db.setUserName("acarlson");
         db.setPassword("1uTbSbAs");
         bool ok = db.open();

The first line creates the connection object, and the last line opens it for use. In between, we initialize some connection information, including the database name, the host name, the user name, and the password. In this case, we are connecting to the MySQL database `flightdb` on the host `bigblue`. The `"QMYSQL"` argument to addDatabase() 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 supported database drivers.

The connection in the snippet will be the default connection, because we don't pass the second argument to addDatabase(), which is the connection name. For example, here we establish two MySQL database connections named `"first"` and `"second"`:

         QSqlDatabase firstDB = QSqlDatabase::addDatabase("QMYSQL", "first");
         QSqlDatabase secondDB = QSqlDatabase::addDatabase("QMYSQL", "second");

After these connections have been initialized, open() for each one to establish the live connections. If the open() fails, it returns false. In that case, call QSqlDatabase::lastError() to get error information.

Once a connection is established, we can call the static function QSqlDatabase::database() 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:

         QSqlDatabase defaultDB = QSqlDatabase::database();
         QSqlDatabase firstDB = QSqlDatabase::database("first");
         QSqlDatabase secondDB = QSqlDatabase::database("second");

To remove a database connection, first close the database using QSqlDatabase::close(), then remove it using the static method QSqlDatabase::removeDatabase().