Qt - 如何将 SQL SELECT COUNT 与参数一起使用

Question

我正在用QT编写程序，但在编写SQL查询Select时遇到问题。我有一个简单的表格，其中包含以下列：ID，name_or_nickname，姓氏，职业。我想在查询中使用 3 个变量：

QString name = "Peter";
QString surname = "Smith"; 
QString occupation = "New York";

我想使用这些变量运行此查询：

 QSqlDatabase sql;                                                 
 QSqlQuery query(sql);
 QString execute = "SELECT COUNT(name) FROM table1 WHERE name_or_nickname='?' AND surname='?' AND occupation='?';";
        query.prepare(execute);
        query.bindValue(0, name);
        query.bindValue(1, _surname);
        query.bindValue(2, _occupation);
        query.exec();
        if (query.next()) 
        {
            rows= query.value(0).toInt();
            return true;
        } 
        else 
        {
            qDebug() << query.lastError();
            return false;
        }

但是它不起作用并且isActive（）函数返回false，因此查询有问题 - 可能是括号。你能给我举个例子吗？提前谢谢你！

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我在这里发布必要的代码：

MainWindow.h：

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <person.h>
#include <QMainWindow>
#include <QString>
#include <QtDebug>
#include <QtSql>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
    Q_OBJECT
public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
private slots:
    void on_pushButton_1_clicked();
private:
    Ui::MainWindow *ui;
    QSqlDatabase sql;
    bool open_or_not;
};
#endif // MAINWINDOW_H

主窗口.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    sql = QSqlDatabase::addDatabase("QSQLITE", "db");
    sql.setDatabaseName("E:\folder\database.sqlite3");
    sql.close(); // this was called beacuse of the problem with first query
    sql.open();
    QSqlQuery query(sql);
    QString execute = "CREATE TABLE IF NOT EXISTS table1 (id INTEGER UNIQUE PRIMARY KEY, name_or_nickname TEXT, surname TEXT, occupation TEXT);";
query.exec(execute);
    qDebug() << "isActive: CREATING TABLE" << query.isActive();
    query.clear();
    open_or_not = sql.open();
}
MainWindow::~MainWindow()
{
    delete ui;
}
void MainWindow::on_pushButton_1_clicked()
{
    if(open_or_not)
    {
        Person person1(_sql, Johnny, Walker, California, this);
        bool result = false;
        result = person1.search_in_database();
        qDebug() << result;
    }
}

人：

#ifndef PERSON_H
#define PERSON_H
#include <QNetworkRequest>
#include <QNetworkReply>
#include <QtDebug>
#include <QDebug>
#include <QJsonDocument>
#include <QtSql>
namespace Ui {
class Person;
}
class Person : public QDialog
{
    Q_OBJECT
public:
    explicit Person(QSqlDatabase & sql, QString name_or_nickname, QString surname, QString occupation, QWidget *parent);
    ~Person();
    bool search_in_table();
private:
    Ui::Person *ui;
    QSqlDatabase sql;
    QString name_or_nickname;
    QString surname;
    QString occupation;

};
#endif // PERSON_H

人.cpp：

#include "person.h"
#include "ui_person.h"

Person::Person(QSqlDatabase & sql, QString name_or_nickname, QString surname, QString occupation, QWidget *parent) :
    QDialog(parent),
    ui(new Ui::Person)
{
    ui->setupUi(this);
    this->sql = sql;
    this->name_or_nickname = name_or_nickname;
    this->surname = surname;
    this->occupation = occupation;
}
Person::~Person()
{
    delete ui;
}
bool Person::search_in_table()
{
    QSqlQuery query(sql);
    int rows = 0;
    QString name = this->name_or_nickname;
    QString _surname = this->surname;
    QString _occupation = this->occupation;
    QString execute = "SELECT COUNT(name) FROM table1 WHERE name_or_nickname='?' AND surname='?' AND occupation='?';";
    query.prepare(execute);
    query.bindValue(0, name);
    query.bindValue(1, _surname);
    query.bindValue(2, _occupation);
    query.exec();
    if (query.next()) 
    {
        rows= query.value(0).toInt();
        return true;
    } 
    else 
    {
        qDebug() << query.lastError();
        return false;
    }
}

主.cpp：

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

Answer 1

最好将查询与参数一起使用，以防止SQL中出现不同类型的转换问题

QString execute = "SELECT COUNT(name) FROM table1 WHERE surname=? AND occupation=?;";
QSqlQuery query;
query.prepare(execute);
query.bindValue(0, surname);
query.bindValue(1, occupation);
query.exec();
if (query.next()) {
    rows= query.value(0).toInt();
} else {
    qDebug() << query.lastError(); //check your error here
}

Answer 2

试试这段代码：

QSqlDatabase _sql;                                                 
QSqlQuery _query(_sql);
QString execute_stat = "SELECT COUNT(name) FROM table1 WHERE surname='%1' AND 
occupation='%2';";
_query.prepare(execute_stat.arg(surname, occupation));
q.exec();
int rows= 0;
if (q.next()) {
    rows= q.value(0).toInt();
}

Answer 3

你忘了在语句末尾加上双引号

你：

QString execute = "SELECT COUNT(name) FROM table1 WHERE surname='%1' AND occupation='%2';

应该是 ：

QString execute = "SELECT COUNT(name) FROM table1 WHERE surname='%1' AND occupation='%2';"

Answer 4

老实说，我不知道为什么上面建议的方法都没有完成这项工作。

但是，最简单的版本对我有用：

Q执行 = "从表 1 中选择 COUNT（*），其中 （name_or_nickname='" + 姓名 + "'） 和 （姓氏='" + 姓氏 + "'）

和 （职业='" + 职业 + "'）;";

谢谢你的帮助。