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create table Spec(code_spec integer not null, code_spec_PNU integer  unique not null, name_spec varchar(200)  unique not null, name_spec_PNU varchar(200)  unique not null, PRIMARY KEY(code_spec)); 
create table Circle(id_circle INTEGER PRIMARY KEY AUTOINCREMENT, name_circle varchar(15)  unique not null); 
create table Disc(id_disc INTEGER PRIMARY KEY AUTOINCREMENT, name_disc varchar(200) unique not null, id_circle references Circle on delete cascade on update cascade); 
create table Norm (id_disc references Disc on delete cascade on update cascade, code_spec references Spec on delete cascade on update cascade, hours integer not null, PRIMARY KEY(id_disc, code_spec));

void MainWindow::on_comboBox_currentTextChanged(const QString &arg1)
{
    QSqlQuery g;
    g.exec("delete from Main");
    g.prepare("INSERT INTO Main SELECT name_circle, name_disc, hours FROM Circle NATURAL JOIN Disc NATURAL JOIN Norm NATURAL JOIN Spec WHERE code_spec=?");
    g.addBindValue(arg1);
    g.exec();
    MainModel->select();
}