In this recipe, we will learn how to connect to our SQL database using Qt's SQL module.
Connecting to SQL server in Qt is really simple:
- First of all, open up Qt Creator and create a new Qt Widgets Application project.
- Open up your project file (
.pro) and add the SQL module to your project, like so:QT += core gui sql
- Next, open up
mainwindow.uiand drag seven label widgets, a combo box, and a checkbox to the canvas. Set the text properties of four of the labels toName:,Age:,Gender:, andMarried:. Then, set theobjectNameproperties of the rest toname,age,gender, andmarried. There is no need to set the object name for the previous four labels because they're for display purposes only:
- After that, open up
mainwindow.hand add the following headers below theQMainWindowheader:#include <QMainWindow> #include <QtSql> #include <QSqlDatabase> #include <QSqlQuery> #include <QDebug>
- Then, open up
mainwindow.cppand insert the following code to the class constructor:MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent), ui(new Ui::MainWindow) { ui->setupUi(this); QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL"); db.setHostName("127.0.0.1"); db.setUserName("yourusername"); db.setPassword("yourpassword"); db.setDatabaseName("databasename"); if (db.open()) { QSqlQuery query; if (query.exec("SELECT name, age, gender, married FROM employee")) { while (query.next()) { qDebug() << query.value(0) << query.value(1) << query.value(2) << query.value(3); ui->name->setText(query.value(0).toString()); ui->age->setText(query.value(1).toString()); ui->gender->setCurrentIndex(query.value(2).toInt()); ui->married->setChecked(query.value(3).toBool()); } } else { qDebug() << query.lastError().text(); } db.close(); } else { qDebug() << "Failed to connect to database."; } } - Compile and run your project now and you should get something like the following:
The previous example shows you how to connect to your SQL database using the QSqlDatabase class derived from the SQL module. You won't be able to access any of the classes related to SQL without adding the module to your Qt project.
We must tell Qt which SQL architecture we are running by mentioning it when calling the addDatabase() function. Options supported by Qt are QSQLITE, QMYSQL, QMYSQL3, QODBC, QODBC3, QPSQL, and QPSQL7
If you encounter an error message that says, QSqlDatabase: QMYSQL driver not loaded, then you should again check whether the DLL files are placed in the correct directory.
We can send our SQL statements to the database through the QSqlQuery class, and wait for it to return the results, which usually are either the data you requested or error messages due to invalid statements.
If there is any data coming from the database server, it will all be stored in the QSqlQuery class. All you need to do to retrieve this data is to do a while loop on the QSqlQuery class to check for all existing records, and retrieve them by calling the value() function.