c++ - 如何在 Postgresql for C++ 中准备语句和绑定(bind)参数

Question

我是 C++ 的新手，对 pqxx 库略知一二。我要实现的是准备语句和绑定(bind)参数。在 PHP 中，我习惯以如此简洁明了的方式执行此操作:

$s = $db->prepare("SELECT id FROM mytable WHERE id = :id");
$s->bindParam(':id', $id);
$s->execute();

或使用代币:

$data = array();
$data[] = 1;
$data[] = 2;
$s = $db->prepare("SELECT id FROM mytable WHERE id = ? or id = ?");
$s->execute($data);

我试图从 pqxx documentation 中逃脱如何实现这一点，但对我来说，文档看起来一团糟，并且缺乏简短的示例(如我上面提供的)。我希望有人在处理 C++ 中的 Postgresql 时也能提供这样简单的示例(或相当简单 - 无需编写一些庞大的代码)。

Answer 1

一个简单的例子。这只会打印 id 值为 0 的条目数。

#include<pqxx/pqxx>
#include<iostream>

int main()
{
    std::string name = "name";
    int id = 0;
    try {
        //established connection to data base
        pqxx::connection c("dbname=mydb user=keutoi");
        pqxx::work w(c);
        //statement template
        c.prepare("example", "SELECT id  FROM mytable WHERE id = $1");
        //invocation as in varible binding
        pqxx::result r = w.prepared("example")(id).exec();
        
        w.commit();
        //result handling for accessing arrays and conversions look at docs
        std::cout << r.size() << std::endl;
    }
    catch(const std::exception &e)
    {
        std::cerr << e.what() << std::endl;
        return 1;
    }
    return 0;
}

函数 w.prepared() 有点复杂。它类似于 haskell 中的 curried(curry) 函数，因为它接受一个参数并返回另一个函数，该函数又接受另一个参数。那种事。

文档说:

How do you pass those parameters? C++ has no good way to let you pass an unlimited, variable number of arguments to a function call, and the compiler does not know how many you are going to pass. There's a trick for that: you can treat the value you get back from prepared as a function, which you call to pass a parameter. What you get back from that call is the same again, so you can call it again to pass another parameter and so on.

Once you've passed all parameters in this way, you invoke the statement with the parameters by calling exec on the invocation

如果有更多参数，请在 prepare 函数中使用 $1 $2 等。

c.prepare("SELECT id name FROM mytable WHERE id = $1 AND name = $2")

并将变量作为

w.prepared("example")(dollar1_var)(dollar2_var).exec()

动态准备的例子

#include<pqxx/pqxx>
#include<iostream>
#include<vector>

//Just give a vector of data you can change the template<int> to any data type
pqxx::prepare::invocation& prep_dynamic(std::vector<int> data, pqxx::prepare::invocation& inv)
{
    for(auto data_val : data)
        inv(data_val);
    return inv;
}

int main()
{
    std::string name = "name";

    //a data array to be used.
    std::vector<int> ids;
    ids.push_back(0);
    ids.push_back(1);

    try {
        pqxx::connection c("dbname=mydb user=keutoi");
        pqxx::work w(c);

        c.prepare("example", "SELECT id  FROM mytable WHERE id = $1 or id = $2");
        pqxx::prepare::invocation w_invocation = w.prepared("example");

        //dynamic array preparation
        prep_dynamic(ids, w_invocation);
        //executing prepared invocation.
        pqxx::result r = w_invocation.exec();

        w.commit();

        std::cout << r.size() << std::endl;
    }
    catch(const std::exception &e)
    {
        std::cerr << e.what() << std::endl;
        return 1;
    }
    return 0;
}

如果你想处理其他数据类型使用这个函数定义

template<class T> pqxx::prepare::invocation& prep_dynamic(std::vector<T> data, pqxx::prepare::invocation& inv)
{
    for(auto data_val : data)
        inv(data_val);
    return inv;
}