why i receive Undefined reference error in QtCreator?

Question

after reading Qt Assistant i make these objects

#include "account.h"
#include<QtSql/QSqlDatabase>
#include<QtSql/QSqlQuery>
Account::Account(QWidget *parent)
: QMainWindow(parent)
{

QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
QSqlQuery q(db);
q.exec("CREATE TABLE my");
}

after running i get these errors:

error: undefined reference to `QSqlDatabase::defaultConnection'

error: undefined reference to `QSqlDatabase::addDatabase(QString const&, QString const&)'
error: undefined reference to `QSqlQuery::QSqlQuery(QSqlDatabase)'

 error: undefined reference to `QSqlQuery::exec(QString const&)'

and other errors about destructors:

error: undefined reference to `QSqlDatabase::~QSqlDatabase()'
.........

Answer 1

Add this in your pro file:

QT += sql