LNK 2019: unresolved external symbol "__declspec(dllimport) public: void__thiscall

[lilgoldfish]

Error:
http://puu.sh/3m6Qr.png

my files

.pro
@#-------------------------------------------------

Project created by QtCreator 2013-06-22T00:33:21

#-------------------------------------------------

QT += core gui
QT += widgets
QT += network

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = guangdong
TEMPLATE = app

SOURCES += main.cpp
login.cpp

HEADERS += login.h

FORMS += login.ui
@

login.cpp
@#include "login.h"
#include "ui_login.h"
#include <QtSql/QSql>
#include <QtSql/QSqlDatabase>
#include <QDebug>
#include <Query.h>
#include <QString>
#include <QtSql/QSqlQuery>
#include <QtNetwork/QNetworkInterface>

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

login::~login()
{
delete ui;
}
void login::blah()
{
// QSqlQuery query;
QSqlDatabase db = QSqlDatabase::addDatabase("QMYSQL");
db.setHostName("blah");
db.setDatabaseName("blah");
db.setUserName("blah");
db.setPassword("blah");
bool ok = db.open();

if ( ok ) {
    ui->label->setText("databaseopen");
    db.close();
}
else
    ui->label->setText("Error opening");

}
@

main.cpp
@#include "login.h"
#include <QApplication>

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
login w;
w.show();

return a.exec();

}
@

I'm not an expert programmer at all, please answer me like i dont know anything, thank you!

Edit: I am using Qt 5.0.2 and msvc2010 compiler

[tobias.hunger]

Try adding Qt += sql into your .pro file.