qt登录界面简单制作，是真的保姆级别了！！！

输入：show databases;

创建一个数据库：create database dbtest;

进入数据库：use dbtest(数据库名);

使用查看当前数据库的表：show tables;
我们这个qt要用到下面的这个表如果没有就用：create table user(username varchar(8),password varchar(8)); 

insert into user(username,password) values('user01','abccba');
insert into user(username,password) values('1','21212121');
然后查看表：select * from user (表名);

QT       += core gui sql
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
CONFIG += c++11
# The following define makes your compiler emit warnings if you use
# any Qt feature that has been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS
# You can also make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0
SOURCES += \
    main.cpp \
    mainwindow.cpp
HEADERS += \
    mainwindow.h
FORMS += \
    mainwindow.ui
# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
    bool connectDB();
private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

#include "mainwindow.h"
#include <QApplication>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QSqlDatabase>
#include <QMessageBox>
#include <QSqlError>
#include <QDebug>
#include <QSqlQueryModel>
#include <QComboBox>
#include <QSqlQuery>
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    connectDB();//调用数据库
    //我们点击之后信号的触发
    connect(ui->pushButton,&QPushButton::clicked,this,[=](){
        QSqlQuery query1;
        query1.exec("SELECT * FROM user WHERE username");//选择我们的表
        QVariantList userlista;
        QString usert;
        usert=ui->lineEdit->text();//将我们ui界面输入框的值传递到usert里面
        QString passwdt;
        passwdt=ui->lineEdit_2->text();//将我们ui界面输入框的值传递到passwdt里面
        bool T1=false;//设立判断标志
        while(query1.next()){//这个回遍历数据库的值
        qDebug()<<query1.value(0).toString();//在qt界面显示
        qDebug()<<query1.value(1).toString();//在qt界面显示
        if(query1.value(0).toString()==usert&&query1.value(1).toString()==passwdt){
           T1=true;//判断密码和账号是否一致
        }
        }
        if(T1==true){
            QMessageBox::information(this, "成功", "登陆成功");
        }
            else { QMessageBox::information(this, "警告", "用户名或密码错误");}
         query1.execBatch();//再次循环，没有这个你的按钮按一次就没有用了
           });
}
bool MainWindow::connectDB(){
   QSqlDatabase db=QSqlDatabase::addDatabase("QMYSQL");
   db.setPort(3306);//你的地址
   db.setHostName("localhost");
   db.setUserName("root");//你的my sql的账号
   db.setPassword("123456");//你进去时候输入的密码
   db.setDatabaseName("dbtest"); //数据库名
   if(!db.open()){
          QMessageBox::critical(this,"数据库打开失败",db.lastError().text());
          return false;
      }
      else {
          return true;
      }
   }
MainWindow::~MainWindow()
{
    delete ui;
}