在Mat的一点帮助下，我能够找出问题所在，但是由于他没有给出答案，我必须回答它，以便可以将这些问题共享给有相同问题的人员，也可以将其标记为已回答.

With a little help from Mat, I was able to figure out what the problem was, but since he didn't give it in a form of an answer, I'll have to answer it so it can be shared for those who have the same problem, and also to mark as answered.

所以，我的问题是我无法连接到数据库.正如Mat所建议的，我应该使用称为SQLGetDiagRec的扩展错误信息，并根据文档修复参数.花了一点时间来学习SQLGetDiagRec函数的工作原理，但是一旦我设法将wchar_t转换为char *，我就能够看到它所产生的错误.

So, my problem was that I couldn't connect to the database. As Mat suggested, I should use the extended error info, known as SQLGetDiagRec and also fix the arguments according to the documentation. Took me a moment to learn how the SQLGetDiagRec function works, but once I managed to convert the wchar_t to char * I was able to see the error it was generating.

连接尝试给了我错误Data source not found and no default driver specified.这给了我一个提示，表明我要么写了错误的连接字符串，要么以某种方式误解了文本字符串.

The connection attempt gave me the error Data source not found and no default driver specified. That gave me a clue, indicating I either wrote the incorrect connection string or that the text string was somehow misinterpreted or mangled.

做一些在网上搜索给了我一个深刻的印象，即字符串被误解了，要修复它，我必须将其设置为文字字符串.果然，在字符串前面加一个L即可解决问题！

Doing some searching on the net gave me the insight that the string was misinterpreted, and to fix it I had to make it a literal string. Surely enough, putting an L in front of the string solved it!

retcode = SQLDriverConnect(hdbc, 0, 
                           (SQLWCHAR*)L"DSN=TestConnection;SERVER=localhost;UID=user;PWD=password;DRIVER=MySQL Server;", 
                           _countof(L"DSN=TestConnection;SERVER=localhost;UID=user;PWD=password;DRIVER=MySQL Server;"), 
                           OutConnStr, 255, &OutConnStrLen, SQL_DRIVER_COMPLETE);

与此同时，我学习了如何摆脱提示，在纠正最初的问题后，很容易找出提示.为窗口句柄指定null，将驱动程序完成设置为SQL_DRIVER_COMPLETE，并确保在连接字符串中添加了所有需要的信息.

At the same time, I learned how to get rid of the prompt, which was quite easy to figure out after correcting the initial problem. Specify null for the window handle, set driver completion to SQL_DRIVER_COMPLETE and make sure you add all the information needed in the connection string.

因此，我对SQLExecDirect的查询遇到的下一个问题是给出错误消息Syntax error or access violation.问题显然与连接字符串相同.果然

So, the next problem I had with the query with SQLExecDirect was giving an error saying Syntax error or access violation. The problem was obviously the same as with the connection string. Surely enough

retcode = SQLExecDirect(hstmt, (SQLWCHAR*)L"SELECT TEST_STRING, TEST_INTEGER, TEST_FLOAT FROM dbo.testfire", SQL_NTS);

像护身符一样工作.

这是完整的，功能齐全的代码:

Here's the code in its entirety, fully functional:

#include <iostream>
#include <windows.h>
#include <sql.h>
#include <sqltypes.h>
#include <sqlext.h>
#include <string>

using namespace std;

int main(){
    SQLHENV henv;
    SQLHDBC hdbc;
    SQLHSTMT hstmt;
    SQLRETURN retcode;

    SQLWCHAR OutConnStr[255];
    SQLSMALLINT OutConnStrLen;

    // Allocate environment handle
    retcode = SQLAllocHandle(SQL_HANDLE_ENV, SQL_NULL_HANDLE, &henv);

    // Set the ODBC version environment attribute
    if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
        retcode = SQLSetEnvAttr(henv, SQL_ATTR_ODBC_VERSION, (void*)SQL_OV_ODBC3, 0); 

        // Allocate connection handle
        if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
            retcode = SQLAllocHandle(SQL_HANDLE_DBC, henv, &hdbc); 

             // Set login timeout to 5 seconds
            if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
                SQLSetConnectAttr(hdbc, SQL_LOGIN_TIMEOUT, (SQLPOINTER)5, 0);

                // Connect to data source
                retcode = SQLDriverConnect(
                    hdbc, 
                    0,
                    (SQLWCHAR*)L"DSN=TestConnection;SERVER=localhost;UID=root;PWD=never140;DRIVER=MySQL Server;", 
                    _countof(L"DSN=TestConnection;SERVER=localhost;UID=root;PWD=never140;DRIVER=MySQL Server;"),
                    OutConnStr,
                    255, 
                    &OutConnStrLen,
                    SQL_DRIVER_COMPLETE );

                // Allocate statement handle
                if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO) {
                    retcode = SQLAllocHandle(SQL_HANDLE_STMT, hdbc, &hstmt); 

                    // Process data
                    retcode = SQLExecDirect(hstmt, (SQLWCHAR*)L"SELECT TEST_STRING, TEST_INTEGER, TEST_FLOAT FROM dbo.testfire", SQL_NTS);

                    if (retcode == SQL_SUCCESS) {
                        SQLINTEGER sTestInt, cbTestStr, cbTestInt, cbTestFloat, iCount = 1;
                        SQLFLOAT dTestFloat;
                        SQLCHAR szTestStr[200];
                        while (TRUE) {
                            retcode = SQLFetch(hstmt);
                            if (retcode == SQL_ERROR || retcode == SQL_SUCCESS_WITH_INFO) {
                                cout<<"An error occurred";
                            }
                            if (retcode == SQL_SUCCESS || retcode == SQL_SUCCESS_WITH_INFO){

                                SQLGetData(hstmt, 1, SQL_C_CHAR, szTestStr, 200, &cbTestStr);
                                SQLGetData(hstmt, 2, SQL_C_ULONG, &sTestInt, 0, &cbTestInt);
                                SQLGetData(hstmt, 3, SQL_C_DOUBLE, &dTestFloat, 0,&cbTestFloat);

                                /* Print the row of data */
                                cout<<"Row "<<iCount<<":"<<endl;
                                cout<<szTestStr<<endl;
                                cout<<sTestInt<<endl;
                                cout<<dTestFloat<<endl;
                                iCount++;
                            } else {
                                break;
                            }
                        }
                    }else{
                        cout<<"Query execution error."<<endl;
                    }

                    SQLFreeHandle(SQL_HANDLE_STMT, hstmt);
                    SQLDisconnect(hdbc);
                }else{ 
                    cout<<"Connection error"<<endl;
                }
                SQLFreeHandle(SQL_HANDLE_DBC, hdbc);
            }
        }
        SQLFreeHandle(SQL_HANDLE_ENV, henv);
    }

        system("pause");
    return 0;
}

只是展示一下，即使是最微小的事情也可以使一切失败.

Just goes to show, even the tiniest thing can make everything fail.

感谢Mat的帮助.