SQLite database does not open

Question

When I run my program, the console says that db is not open and tableView does not include a thing:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include "QDebug"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName("./DB_gym.db");
    query = new QSqlQuery(db);
    query->exec("CREATE TABLE asd(first TEXT, second TEXT, third INT);");
    model = new QSqlTableModel(this, db);
    model->setTable("asd");
    model->select();
    ui->tableView->setModel(model);
}

MainWindow::~MainWindow()
{
    delete ui;
}

Headers file

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QSqlDatabase>
#include <QSqlQuery>
#include <QSqlTableModel>
#include <QTableView>
#include <QGridLayout>
#include <QMessageBox>

QT_BEGIN_NAMESPACE
namespace Ui {
class MainWindow;
}
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
    QSqlDatabase db;
    QSqlQuery *query;
    QSqlTableModel *model;
    QMessageBox *msg;
};
#endif // MAINWINDOW_H

Why db is not opened?