mysql cast函数REAL

MySQL中的CAST函数用于将一个表达式转换为特定的数据类型。对于将一个值转换为REAL类型，你可以使用CAST函数并指定REAL作为目标数据类型。

在MySQL中，REAL类型对应于单精度浮点数，它具有4个字节的存储空间，可以表示小数位数较少的浮点数。

下面是使用CAST函数将一个值转换为REAL类型的示例：

SELECT CAST('3.14' AS REAL);

这将返回3.14作为REAL类型的结果。

请注意，如果要将一个列或表达式的数据类型更改为REAL类型，可以在ALTER TABLE语句中使用ALTER COLUMN子句来更改列的数据类型。例如：

ALTER TABLE your_table MODIFY your_column REAL;

这将把`your_table`表中的`your_column`列的数据类型更改为REAL类型。

相关问题

mysql自定义聚合函数

MySQL 自定义聚合函数是在标准函数的基础上，允许用户创建满足特定业务场景需求的计算功能。创建自定义聚合函数通常涉及编写存储过程或创建用户定义函数（UDF）。以下是基本流程：

1. **创建存储过程**[^1]:

   CREATE PROCEDURE custom_sum (IN input_column VARCHAR(255), OUT result INT)
   BEGIN
     SELECT SUM(CAST(input_column AS REAL)) INTO result FROM table_name;
   END;

2. **创建用户定义函数 (UDF)**[^2]:

   DELIMITER //
   CREATE FUNCTION custom_agg_func(param1 INT, param2 INT)
       RETURNS INT
   DETERMINISTIC
   BEGIN
      RETURN param1 + param2;
   END; 
   //DELIMITER ;

3. **调用自定义函数**:

   SELECT custom_agg_func(column1, column2) FROM table_name;

要记住，自定义函数可能需要在每次查询时显式调用，因为它不像内置函数那样自动执行。

修改以下代码使其能够接受视频并保存到本地，上传到数据库void ShowVideo() { //创建数据库连接 MYSQL mysql; mysql_init(&mysql); mysql_real_connect(&mysql, "localhost", "root", "123456", "myserver", 3306, NULL, 0); mysql_query(&mysql, "set names utf8"); SOCKET server = socket(AF_INET, SOCK_STREAM, 0); char buffer[2048] = " "; SOCKET sockConn = accept(server, NULL, NULL); Mat videoData; while (true) { int bytes = recv(sockConn, buffer, 2048, 0); if (bytes <= 0) break; // 将接收到的视频数据拼接在一起 videoData.push_back(Mat(1, bytes, CV_8UC1, buffer).clone()); } closesocket(sockConn); //size_t fileSize = 0; std::ofstream videoFile("server_video.avi", std::ios::binary | std::ios::app); while (true) { int bytes = recv(sockConn, buffer, 2048, 0); if (bytes <= 0) break; //fileSize += bytes; // 将接收到的视频数据写入到.avi文件中 //std::ofstream videoFile("server_video.avi", std::ios::binary | std::ios::app); videoFile.write(buffer, bytes); } videoFile.close(); VideoWriter writer("server_video.avi", cv::VideoWriter::fourcc('M', 'J', 'P', 'G'), 25.0, Size(640, 480)); if (writer.isOpened()) { //writer.write(videoData); cout << "打开失败" << endl; return; } //读取视频文件内容并保存到数据库 ifstream fin("server_video.avi", ios::binary | ios::in); if (!fin) { cout << "打开文件失败" << endl; exit(0); } fin.seekg(0, ios::end); int filesize = fin.tellg(); fin.seekg(0, ios::beg); char* fileContent = new char[filesize]; fin.read(fileContent, filesize); string fileContentStr(fileContent, filesize); //将视频内容上传到数据库 string strSql = "INSERT INTO videos(content) VALUES('"+ fileContentStr + "')"; mysql_query(&mysql, strSql.c_str()); delete[] fileContent; fin.close(); mysql_close(&mysql); closesocket(sockConn); closesocket(server); WSACleanup(); }

注：为了确保代码的正确性和安全性，以下是修改后的代码：

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <sstream>
#include <string>
#include <fstream>
#include <vector>
#include <winsock.h>
#include <mysql.h>
#include <opencv2/opencv.hpp>

#pragma comment(lib, "ws2_32.lib")
#pragma comment(lib, "libmysql.lib")

void SaveVideoToDatabase(const std::string& videoPath, MYSQL* mysql) {
    std::ifstream file(videoPath, std::ios::in | std::ios::binary);
    if (file) {
        file.seekg(0, std::ios::end);
        std::streampos fileSize = file.tellg();
        file.seekg(0, std::ios::beg);

        std::vector<char> videoData(fileSize);
        file.read(videoData.data(), fileSize);
        file.close();

        std::string videoDataStr(videoData.data(), fileSize);

        std::string strSql = "INSERT INTO videos(content) VALUES('" + videoDataStr + "')";
        if (mysql_query(mysql, strSql.c_str()) != 0) {
            std::cerr << "Failed to save video to database." << std::endl;
        }
        else {
            std::cout << "Video saved to database successfully." << std::endl;
        }
    }
    else {
        std::cerr << "Failed to open video file." << std::endl;
    }
}

void ShowVideo() {
    WSADATA wsaData;
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
        std::cerr << "Failed to initialize winsock." << std::endl;
        return;
    }

    SOCKET serverSocket = socket(AF_INET, SOCK_STREAM, 0);
    if (serverSocket == INVALID_SOCKET) {
        std::cerr << "Failed to create socket." << std::endl;
        WSACleanup();
        return;
    }

    sockaddr_in serverAddress{};
    serverAddress.sin_family = AF_INET;
    serverAddress.sin_port = htons(12345); // 修改为服务端的端口号
    serverAddress.sin_addr.s_addr = htonl(INADDR_ANY);

    if (bind(serverSocket, reinterpret_cast<sockaddr*>(&serverAddress), sizeof(serverAddress)) != 0) {
        std::cerr << "Failed to bind socket." << std::endl;
        closesocket(serverSocket);
        WSACleanup();
        return;
    }

    if (listen(serverSocket, 1) != 0) {
        std::cerr << "Failed to listen on socket." << std::endl;
        closesocket(serverSocket);
        WSACleanup();
        return;
    }

    std::cout << "Waiting for client connection..." << std::endl;

    SOCKET clientSocket = accept(serverSocket, NULL, NULL);
    if (clientSocket == INVALID_SOCKET) {
        std::cerr << "Failed to accept client connection." << std::endl;
        closesocket(serverSocket);
        WSACleanup();
        return;
    }

    std::vector<char> videoData;
    char buffer[2048];
    int bytesRead = 0;
    while ((bytesRead = recv(clientSocket, buffer, sizeof(buffer), 0)) > 0) {
        videoData.insert(videoData.end(), buffer, buffer + bytesRead);
    }

    closesocket(clientSocket);
    closesocket(serverSocket);
    WSACleanup();

    std::string videoPath = "server_video.avi"; // 修改为要保存的视频文件路径
    std::ofstream videoFile(videoPath, std::ios::binary);
    if (videoFile) {
        videoFile.write(videoData.data(), videoData.size());
        videoFile.close();
        std::cout << "Video saved successfully." << std::endl;

        MYSQL mysql;
        mysql_init(&mysql);
        if (mysql_real_connect(&mysql, "localhost", "root", "123456", "myserver", 3306, NULL, 0) != NULL) {
            std::cout << "Connected to MySQL database." << std::endl;
            SaveVideoToDatabase(videoPath, &mysql);
            mysql_close(&mysql);
        }
        else {
            std::cerr << "Failed to connect to MySQL database." << std::endl;
        }
    }
    else {
        std::cerr << "Failed to save video." << std::endl;
    }
}

请确保以下几点：

1. 修改 `serverAddress.sin_port` 为服务端的端口号。
2. 修改 `videoPath` 为要保存的视频文件路径。
3. 修改 `mysql_real_connect` 函数的参数来连接到正确的 MySQL 数据库。

注意：这是一个简单的示例代码，仅展示了如何接收视频数据并将其保存到本地和上传到数据库。实际情况可能需要进行更多的错误处理和安全性考虑。